View source for Solar water heating

{{Short description|Use of sunlight for water heating with a solar thermal collector}}
[[File:Water collectors.jpg|thumb|Solar water collectors installed in [[Spain]]]]

<noinclude>{{Sustainable energy}}</noinclude>
'''Solar water heating''' ('''SWH''') is [[water heating|heating water]] by [[sunlight]], using a [[solar thermal collector]]. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. SWHs are widely used for residential and some industrial applications.<ref>{{Cite web |date=2013-01-28 |title=Solar Collectors ...Behind the Glass {{!}} Home Power Magazine |url=https://www.homepower.com/articles/solar-collectors-behind-glass |access-date=2022-08-04 |archive-url=https://web.archive.org/web/20130128023233/https://www.homepower.com/articles/solar-collectors-behind-glass |archive-date=2013-01-28 }}</ref><ref>{{Cite web |title=Solar Water Heating |url=https://www.nrel.gov/research/re-solar-water-heating.html |access-date=2023-10-05 |website=www.nrel.gov |language=en}}</ref>

A Sun-facing collector heats a [[working fluid]] that passes into a storage system for later use. SWH are active (pumped) and passive ([[Natural convection|convection]]-driven). They use water only, or both water and a working fluid. They are heated directly or via light-concentrating mirrors. They operate independently or as hybrids with electric or gas heaters.<ref>Brian Norton (2011) Solar Water Heaters: A Review of Systems Research and Design Innovation, Green. 1, 189–207, ISSN (Online) 1869-8778</ref> In large-scale installations, mirrors may concentrate sunlight into a smaller collector.{{Original research inline|date=July 2022}}

At the end of 2023, global solar hot water thermal capacity was 560 [[GWth|GW<sub>th</sub>]], a 3% increase from 2022.<ref name="iea2022" /> The market is dominated by [[Solar power in China|China]], the [[Solar power in the United States|United States]] and [[Solar power in Turkey|Turkey]].<ref name=ren21_2019>{{cite web |title=Renewables Global Status Report|url=http://www.ren21.net/status-of-renewables/global-status-report/ |publisher=[[REN21]] |access-date=11 May 2019}}</ref> [[Barbados]], [[Austria]], [[Cyprus]], [[Israel]] and [[Greece]] are the leading countries by capacity per person.<ref name=ren21_2019/> There were 122 million solar hot water systems in operation at the end of 2022.<ref name="iea2022" />

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==History==
[[File:Out west (1902) - Solar water heater advert.jpg|thumb|left|An advertisement for a '''Solar Water Heater''' dating to 1902]]
[[File:The Electrical Experimenter, Volume 3.pdf|thumb|page=643|100px|[[Frank Shuman]]'s sunengine on the March 1916 cover of [[Hugo Gernsback|Hugo Gernsback's]] ''[[The Electrical Experimenter]]'']]
Records of solar collectors in the United States date to before 1900,<ref name="CSC">{{usurped|1=[https://web.archive.org/web/20011030174236/http://www.californiasolarcenter.org/history_solarthermal.html Solar Evolution – The History of Solar Energy]}}, John Perlin, California Solar Center</ref> involving a black-painted tank mounted on a roof. In 1896 Clarence Kemp of Baltimore enclosed a tank in a wooden box, thus creating the first 'batch water heater' as they are known today. [[Frank Shuman]] built the world's first solar thermal power station in [[Maadi, Egypt]] between 1912 and 1913, using [[parabolic trough]]s to power a {{convert|60|to|70|hp|kW|order=flip|abbr=off}} engine that pumped {{convert|6000|usgal|L|order=flip}} of water per minute from the [[Nile|Nile River]] to adjacent cotton fields.<ref>{{cite book|last=Smith|first=Zachary Alden |author2=Taylor, Katrina D.|title=Renewable and alternative energy resources: a reference handbook|url=https://archive.org/details/unset0000unse_z1v3|publisher=[[ABC-CLIO]]|date=2008|page=[https://archive.org/details/unset0000unse_z1v3/page/174 174]|isbn=978-1-59884-089-6}}</ref>

Flat-plate collectors for solar water heating were used in Florida and Southern California in the 1920s. Interest grew in North America after 1960, but especially after the [[1973 oil crisis]].{{Citation needed|date=March 2025}}

===Mediterranean===
{{See also|Solar power in Cyprus|Solar power in Greece|Solar power in Israel|Solar power in Italy|Solar power in Spain}}
[[File:Solarboiler.jpg|thumb|Passive ([[thermosiphon]]) solar water heaters on a rooftop in [[Jerusalem]]]]

Israel, Cyprus and Greece are the ''per capita'' leaders in the use of solar water heating systems supporting 30%–40% of homes.<ref name="Environment California SWH">{{cite web|author1=Del Chiaro, Bernadette  |author2=Telleen-Lawton, Timothy  |name-list-style=amp |date=April 2007 |title=Solar Water Heating (How California Can Reduce Its Dependence on Natural Gas) |publisher=Environment California Research and Policy Center |url=https://www.environmentcalifornia.org/uploads/at/56/at563bKwmfrtJI6fKl9U_w/Solar-Water-Heating.pdf |access-date=29 September 2007 |url-status=unfit |archive-url=https://web.archive.org/web/20071021142503/https://www.environmentcalifornia.org/uploads/at/56/at563bKwmfrtJI6fKl9U_w/Solar-Water-Heating.pdf |archive-date=October 21, 2007 }}</ref>

Flat plate solar systems were perfected and used on a large scale in Israel. In the 1950s a fuel shortage led the government to forbid heating water between 10&nbsp;pm and 6&nbsp;am. [[Levi Yissar]] built the first prototype Israeli solar water heater and in 1953 he launched the NerYah Company, Israel's first commercial manufacturer of solar water heating.<ref name="Bach">{{cite book|author=John Christopher Bacher|title=Petrotyranny|url={{google books |plainurl=y |id=P7LPZk8NuBgC|page=70}}|year=2000|publisher=Dundurn|isbn=978-0-88866-956-8|page=70}}</ref> Solar water heaters were used by 20% of the population by 1967. Following the energy crisis in the 1970s, in 1980 Israel required the installation of solar water heaters in all new homes (except high towers with insufficient roof area).<ref>{{cite web|url=http://www.climate.org/topics/international-action/israel-solar.html#i |title=Israel's Solar Industry: Reclaiming a Legacy of Success |publisher=Climate.org |access-date=10 February 2012}}</ref> As a result, Israel became the world leader in the use of solar energy ''per capita'' with 85% of households using solar thermal systems (3% of the primary national energy consumption),<ref>{{cite web|author=Minicy Catom Software Engineering Ltd. www.catom.com |url=http://www.neaman.org.il/neaman/publications/publication_item.asp?fid=590&parent_fid=490&iid=3639 |title=The Samuel Neaman Institute for Advanced Studies in Science and Technology – Publications – Solar energy for the production of heat Summary and recommendations of the 4th assembly of the energy forum at SNI |publisher=Neaman.org.il |access-date=2012-06-23 |url-status=dead |archive-url=https://web.archive.org/web/20120209044245/http://www.neaman.org.il/neaman/publications/publication_item.asp?fid=590&parent_fid=490&iid=3639 |archive-date=February 9, 2012 }}</ref> estimated to save the country {{convert|2|Moilbbl|m3}} of oil a year.<ref name="Grossman">[http://www.ises.org.il/assets/files/ISES%20Info/IsraelSectionISESfinal.pdf Israeli Section of the International Solar Energy Society], edited by Gershon Grossman, Faculty of Mechanical Energy, Technion, Haifa; Final draft.</ref><ref>{{Cite web|date=2020-02-06|title=Solar Hot Water|url=https://drawdown.org/solutions/solar-hot-water|access-date=2020-12-05|website=Project Drawdown|language=en}}</ref>

In 2005, Spain became the world's first country to require the installation of [[photovoltaic]] electricity generation in new buildings, and the second (after Israel) to require the installation of solar water heating systems, in 2006.<ref name="REN21 2009">{{cite web|url=http://www.ren21.net/globalstatusreport/g2009.asp |title=Renewables Global Status Report: Energy Transformation Continues Despite Economic Slowdown |work=ren21.net |date=13 May 2009 |access-date=20 May 2010 |url-status=unfit |archive-url=https://web.archive.org/web/20100209005403/http://www.ren21.net/globalstatusreport/g2009.asp |archive-date=February 9, 2010 }}</ref>

===Asia===
{{See also|Solar power in China}}

[[File:SolarGlobal2007V2.png|thumb|New solar hot water installations during 2009, worldwide]]
After 1960, systems were marketed in Japan.<ref name="CSC" />

Australia has a variety of national and state and regulations for solar thermal starting with MRET in 1997.<ref>{{cite web|url=http://www.docstoc.com/docs/26723075/5-Star-Housing-%E2%80%93-Performance-Based-Building-Regulation-Delivers|title=5 Star Housing – Performance Based Building Regulation Delivers|publisher=Docstoc.com|access-date=10 February 2012}}</ref><ref>{{cite web|url=http://www.environment.gov.au/sustainability/energyefficiency/buildings/homes/index.html|title=Buildings – Think Change|date=1 November 2010|publisher=Environment.gov.au|access-date=10 February 2012|url-status=dead|archive-url=https://web.archive.org/web/20100507211551/http://www.environment.gov.au/sustainability/energyefficiency/buildings/homes/index.html|archive-date=May 7, 2010}}</ref><ref>Israel del Mundo and Ian Wills (2005) [https://web.archive.org/web/20080722132746/http://www.aares.info/files/2005_delmundo.pdf The Economics of the Mandatory Renewable Energy Target (MRET)], Department of Economics Monash University, Australia.</ref>

Solar water heating systems are popular in China, where basic models start at around 1,500 [[Renminbi|yuan]] (US$235), around 80% less than in Western countries for a given collector size. At least 30 million Chinese households have one. The popularity is due to efficient evacuated tubes that allow the heaters to function even under gray skies and at temperatures well below freezing.<ref>[http://www.planetark.org/dailynewsstory.cfm?newsid=36636 Energy-Hungry China Warms to Solar Water Heaters] discusses [[China Himin Solar Energy Group]] in [[Dezhou]]. Reuters article, posted on [[Planet Ark]] site</ref>

==Design requirements==
{{More citations needed section|date=January 2016}}

The type, complexity and size of a solar water heating system is mostly determined by:

* Changes in ambient temperature and solar radiation between summer and winter
* Changes in ambient temperature during the day-night cycle
* Possibility of the potable water or collector fluid overheating or freezing

The minimum requirements of the system are typically determined by the amount or temperature of hot water required during winter, when a system's output and incoming water temperature are typically at their lowest. The maximum output of the system is determined by the need to prevent the water in the system from becoming too hot.

===Freeze protection===

Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid. Drainback systems drain the transfer fluid from the system when the pump stops. Many indirect systems use [[antifreeze]] (e.g., [[propylene glycol]]) in the heat transfer fluid.

In some direct systems, collectors can be manually  drained when freezing is expected. This approach is common in climates where freezing temperatures do not occur often but can be less reliable than an automatic system as it relies on an operator.

The third type of freeze protection is freeze-tolerance, where low-pressure water pipes made of silicone rubber simply expand on freezing. One such collector now has European Solar Keymark accreditation.

Notably, while the need for freeze protection has traditionally been a complicating factor, systems using evacuated tube collectors may be used in climates with moderate frosts without any need for drainback or antifreeze. This is because an evacuated tube system only has water in the thermally insulated header, not in the evacuated tubes themselves. Additional protection is also provided by the controller: if header temperature drops below a set value (typically 2 to 4'C) the circulating pump is automatically turned on for a short period to pass warmer water to the header.

===Overheat protection===

When no hot water has been used for a day or two, the fluid in the collectors and storage can reach high temperatures in all non-"drainback" systems. When the storage tank in a "drainback" system reaches its desired temperature, the pumps stop, ending the heating process and thus preventing the storage tank from overheating.

Some active systems deliberately cool the water in the storage tank by circulating hot water through the collector at times when there is little sunlight or at night, losing heat. This is most effective in direct or thermal store plumbing and is virtually ineffective in systems that use evacuated tube collectors, due to their superior insulation. Any collector type may still overheat. High pressure, sealed solar thermal systems ultimately rely on the operation of [[Safety valve#Water heaters|temperature and pressure relief valves]]. Low pressure, open vented heaters have simpler, more reliable safety controls, typically an open vent.

==Structure and working==
Simple designs include a simple glass-topped insulated box with a flat solar absorber made of dark-colored sheet metal, attached to [[Copper in heat exchangers|copper heat exchanger pipes]], or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In industrial cases a [[Parabolic reflector|parabolic mirror]] can concentrate sunlight on the tube. Heat is stored in a [[hot water storage tank]]. The volume of this tank needs to be larger with solar heating systems to compensate for bad weather{{Clarify|date=March 2012}} and because the optimum final temperature for the solar collector{{Clarify|date=March 2012}} is lower than a typical immersion or combustion heater. The heat transfer fluid (HTF) for the absorber may be  water, but more commonly (at least in active systems) is a separate loop of fluid containing [[anti-freeze]] and a [[corrosion inhibitor]] delivers heat to the tank through a [[heat exchanger]] (commonly a coil of [[Copper in heat exchangers|copper heat exchanger tubing]] within the tank). [[Copper in renewable energy#Solar water heaters (solar domestic hot water systems)|Copper is an important component in solar thermal heating and cooling systems]] because of its high heat conductivity, atmospheric and water corrosion resistance, sealing and joining by soldering and mechanical strength. Copper is used both in receivers and primary circuits (pipes and heat exchangers for water tanks).<ref>2011 global status report by Renewable Energy Policy Network for the 21st Century (REN21)</ref>

The 'drain-back' is another lower-maintenance concept.<ref>{{Cite journal |last1=Botpaev |first1=R. |last2=Louvet |first2=Y. |last3=Perers |first3=B. |last4=Furbo |first4=S. |last5=Vajen |first5=K. |date=2016-04-01 |title=Drainback solar thermal systems: A review |url=https://www.sciencedirect.com/science/article/pii/S0038092X15006039 |journal=Solar Energy |series=Special issue: Progress in Solar Energy |language=en |volume=128 |pages=41–60 |doi=10.1016/j.solener.2015.10.050 |bibcode=2016SoEn..128...41B |s2cid=55264769 |issn=0038-092X}}</ref> No anti-freeze is required; instead, all the piping is sloped to cause water to drain back to the tank. The tank is not pressurized and operates at atmospheric pressure. As soon as the pump shuts off, flow reverses and the pipes empty before freezing can occur.{{Citation needed|date=March 2025}}

[[File:Solar Hot Water Diagram.png|thumb|How a solar hot water system works]]
Residential solar thermal installations fall into two groups: passive (sometimes called "compact") and active (sometimes called "pumped") systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting, ensuring that hot water is always available. The combination of solar water heating and back-up heat from a wood stove chimney<ref>{{cite web|url=http://www.woodheat.org/heating-water-with-a-wood-stove.html|title=Heating water with a wood stove|publisher=Wood Heat Organization Inc.|last1=Gulland|first1=John|work=woodheat.org|access-date=29 March 2012}}</ref> can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.

When a solar water heating and hot-water central heating system are used together, solar heat will either be concentrated in a pre-heating tank that feeds into the tank heated by the [[central heating]], or the solar heat exchanger will replace the lower heating element and the upper element will remain to provide for supplemental heat. However, the primary need for central heating is at night and in winter when solar gain is lower. Therefore, solar water heating for washing and bathing is often a better application than central heating because supply and demand are better matched. In many climates, a solar hot water system can provide up to 85% of domestic hot water energy. This can include domestic non-electric [[concentrating solar thermal]] systems. In many northern European countries, combined hot water and space heating systems ([[solar combisystem]]s) are used to provide 15 to 25% of home heating energy. When combined with [[Seasonal thermal energy storage|storage]], large scale solar heating can provide 50-97% of annual heat consumption for [[district heating]].<ref name="drake1">{{Citation|last=Wong|first=Bill|title=Drake Landing Solar Community|date=June 28, 2011|series=IDEA/CDEA District Energy/CHP 2011 Conference, Toronto|pages=1–30|contribution=Drake Landing Solar Community|contribution-url=http://www.districtenergy.org/assets/pdfs/2011Annual_Conf/Proceedings/A24WONG-v03.pdf|access-date=21 April 2013|archive-date=10 September 2016|archive-url=https://web.archive.org/web/20160910135940/http://www.districtenergy.org/assets/pdfs/2011Annual_Conf/Proceedings/A24WONG-v03.pdf|url-status=dead}}</ref><ref name="ing2015-06-14">{{cite web|url=http://ing.dk/artikel/verdens-stoerste-damvarmelager-indviet-i-vojens-176776|title=Verdens største damvarmelager indviet i Vojens|date=14 June 2015|first1=Sanne|last1=Wittrup|work=[[Ingeniøren]]|url-status=dead|archive-url=https://web.archive.org/web/20151019125824/http://ing.dk/artikel/verdens-stoerste-damvarmelager-indviet-i-vojens-176776|archive-date=19 October 2015}}</ref>

===Heat transfer===

====Direct====
[[File:DirectSolarSystems.jpg|thumb|upright=1.4| Direct systems: (A) Passive CHS system with tank above collector. (B) Active system with pump and controller driven by a photovoltaic panel.]]

''Direct'' or ''open loop'' systems circulate potable water through the collectors. They are relatively cheap. Drawbacks include:
* They offer little or no overheat protection unless they have a heat export pump.
* They offer little or no freeze protection, unless the collectors are freeze-tolerant or are of the evacuated tube type.
* Collectors accumulate [[Fouling|scale]] in hard water areas, unless an ion-exchange softener is used.

The advent of freeze-tolerant designs expanded the market for SWH to colder climates. In freezing conditions, earlier models were damaged when the water turned to ice, rupturing one or more components.{{Citation needed|date=March 2025}}

====Indirect====
''Indirect'' or ''closed loop'' systems use a heat exchanger to transfer heat from the "heat-transfer fluid" (HTF) fluid to the potable water. The most common HTF is an antifreeze/water mix that typically uses non-toxic [[propylene glycol]]. After heating in the panels, the HTF travels to the heat exchanger, where its heat is transferred to the potable water. Indirect systems offer freeze protection and typically overheat protection.{{Citation needed|date=March 2025}}

===Propulsion===

====Passive====

''Passive'' systems rely on heat-driven convection or heat pipes to circulate the working fluid. Passive systems cost less and require low or no maintenance, but are less efficient. Overheating and freezing are major concerns.{{Citation needed|date=March 2025}}

====Active====
''Active'' systems use one or more pumps to circulate water and/or heating fluid''.'' This permits a much wider range of system configurations. Pumped systems are more expensive to purchase and to operate. However, they operate at higher efficiency and can be more easily controlled. Active systems have controllers with features such as interaction with a backup electric or gas-driven water heater, calculation and logging of the energy saved, safety functions, remote access and informative displays.{{Citation needed|date=March 2025}}

===Passive direct systems===
[[File:Batch solar thermal collectorColour.jpg|thumb|An integrated collector storage (ICS) system]]

An ''integrated collector storage'' (ICS or batch heater) system uses a tank that acts as both storage and collector. Batch heaters are thin rectilinear tanks with a glass side facing the Sun at [[Solar noon|noon]]. They are simple and less costly than plate and tube collectors, but they may require bracing if installed on a roof (to support {{convert|400|-|700|lb|kg|abbr=on}}&nbsp;lbs of water), suffer from significant heat loss at night since the side facing the sun is largely uninsulated and are only suitable in moderate climates.

A ''convection heat storage unit'' (CHS) system is similar to an ICS system, except the storage tank and collector are physically separated and transfer between the two is driven by convection. CHS systems typically use standard flat-plate type or evacuated tube collectors. The storage tank must be located above the collectors for convection to work properly. The main benefit of CHS systems over ICS systems is that heat loss is largely avoided since the storage tank can be fully insulated. Since the panels are located below the storage tank, heat loss does not cause convection, as the cold water stays at the lowest part of the system.{{Citation needed|date=March 2025}}

===Active indirect systems===

''Pressurized antifreeze'' systems use a mix of [[antifreeze]] (almost always low-toxic propylene glycol) and water mix for HTF in order to prevent freeze damage.

Though effective at preventing freeze damage, antifreeze systems have drawbacks:
* If the HTF gets too hot the glycol degrades into acid and then provides no freeze protection and begins to dissolve the solar loop's components.
* Systems without drainback tanks must circulate the HTF – regardless of the temperature of the storage tank – to prevent the HTF from degrading. Excessive temperatures in the tank cause increased scale and sediment build-up, possible severe burns if a tempering valve is not installed, and if used for storage, possible thermostat failure.
* The glycol/water HTF must be replaced every 3–8 years, depending on the temperatures it has experienced.
* Some jurisdictions require more-expensive, double-walled heat exchangers even though propylene glycol is low-toxic.
* Even though the HTF contains glycol to prevent freezing, it circulates hot water from the storage tank into the collectors at low temperatures (e.g. below {{convert|40|F|C}}), causing substantial heat loss.

A ''drainback system'' is an active indirect system where the HTF (usually pure water) circulates through the collector, driven by a pump. The collector piping is not pressurized and includes an open drainback reservoir that is contained in conditioned or semi-conditioned space. The HTF remains in the drainback reservoir unless the pump is operating and returns there (emptying the collector) when the pump is switched off. The collector system, including piping, must drain via gravity into the drainback tank. Drainback systems are not subject to freezing or overheating. The pump operates only when appropriate for heat collection, but not to protect the HTF, increasing efficiency and reducing pumping costs.<ref name="Lane&Olson">{{cite journal|author1=Lane, T.  |author2=Olson, K.  |name-list-style=amp |year=2002|title= Solar hot water for cold climates: Part II – Drainback systems|journal=Homepower Magazine|volume=86|pages=62–70}}</ref>

===Do-it-yourself (DIY)===
Plans for solar water heating systems are available on the Internet.<ref>{{cite web|url=http://dmoztools.net/Science/Technology/Energy/Renewable/Solar/Solar_Thermal/|title=DMOZ DIY Solar water heating collector|date=2010-05-03|publisher=Dmoz.org|access-date=2012-06-23}}</ref> DIY SWH systems are usually cheaper than commercial ones, and they are used both in the developed and developing world.<ref>{{cite web|url=http://practicalaction.org/practicalanswers/product_info.php?cPath=21_59&products_id=174|title=DIY solar water heating in the developing world|publisher=Practicalaction.org|author=Technical Information Online|access-date=2012-06-23}}</ref><ref>{{cite web|url=https://practicalaction.org/practicalanswers/product_info.php?cPath=21_59&products_id=174|title=archive|archive-url=https://web.archive.org/web/20110207140926/https://practicalaction.org/practicalanswers/product_info.php?cPath=21_59&products_id=174 |archive-date=2011-02-07 }}</ref>

===Comparison===
{| class="wikitable" style="text-align: center;"
|-
! Characteristic
! ICS (Batch)
! Thermo{{shy}}siphon
! Active direct
! Active indirect
! Drain{{shy}}back
! Bubble pump
|-
! Low profile-unobtrusive
|
|
| {{y}}
| {{y}}
| {{y}}
| {{y}}
|-
! Lightweight collector
|
|
| {{y}}
| {{y}}
| {{y}}
| {{y}}
|-
! Survives freezing weather
|
|
| {{y}}
| {{y}}
| {{y}}
| {{y}}
|-
! Low maintenance
| {{y}}
| {{y}}
| {{y}}
|
| {{y}}
| {{y}}
|-
! Simple: no ancillary control
| {{y}}
| {{y}}
|
|
|
| {{y}}
|-
! Retrofit potential to existing store
|
|
| {{y}}
| {{y}}
| {{y}}
| {{y}}
|-
! Space saving: no extra storage tank
| {{y}}
| {{y}}
|
|
|
|
|-
! colspan="7" style="font-size: 0.85em; font-weight: normal; padding: 6px 4px; text-align: left; " | Comparison of SWH systems. Source: ''Solar Water Heating Basics—homepower.com''<ref>{{cite web |url=http://www.homepower.com/articles/solar-water-heating/basics/what-solar-water-heating |title=Solar Water Heating Basics |work=homepower.com |access-date=August 1, 2015}}</ref>
|}

==Components==

===Collector===
{{Main|Solar thermal collector}}
Solar thermal collectors capture and retain heat from the sun and use it to heat a liquid.<ref>{{cite book|last=Norton|first=Brian|title=Harnessing Solar Heat|year=2013|publisher=Springer|isbn=978-94-007-7275-5|url={{google books |plainurl=y |id=3hLHBAAAQBAJ}}}}</ref> Two important physical principles govern the technology of solar thermal collectors:
* Any hot object ultimately returns to thermal equilibrium with its environment, due to heat loss from [[Thermal conduction|conduction]], [[convection]] and radiation.<ref name="W.M. Rohsenow, J.P. Harnett 1998">W.M. Rohsenow, J.P. Harnett, Y.I. Cho (1998). ''Handbook of heat transfer 3rd Ed.''. McGraw-Hill, Chicago, US.</ref> Efficiency (the proportion of heat energy retained for a predefined time period) is directly related to heat loss from the collector surface. Convection and radiation are the most important sources of heat loss. [[Thermal insulation]] is used to slow heat loss from a hot object. This follows  the [[Second law of thermodynamics]] (the 'equilibrium effect').
* Heat is lost more rapidly if the temperature difference between a hot object and its environment is larger. Heat loss is predominantly governed by the thermal gradient between the collector surface and the ambient temperatures. Conduction, convection and radiation all occur more rapidly over large thermal gradients<ref name="W.M. Rohsenow, J.P. Harnett 1998" /> (the delta-''t'' effect).

[[File: Flat-plate solar thermal collector, viewed from roof-level.png|thumb|Flat-plate solar thermal collector, viewed from roof-level]]

====Flat plate solar ====
Flat plate collectors are an extension of the idea to place a collector in an 'oven'-like box with glass directly facing the Sun.<ref name="C. Marken 2009" /> Most flat plate collectors have two horizontal pipes at the top and bottom, called headers, and many smaller vertical pipes connecting them, called risers. The risers are welded (or similarly connected) to thin absorber fins. Heat-transfer fluid (water or water/antifreeze mix) is pumped from the hot water storage tank or heat exchanger into the collectors' bottom header, and it travels up the risers, collecting heat from the absorber fins, and then exits the collector out of the top header. Serpentine flat plate collectors differ slightly from this "harp" design, and instead use a single pipe that travels up and down the collector. However, since they cannot be properly drained of water, serpentine flat plate collectors cannot be used in drainback systems.

The type of glass used in flat plate collectors is almost always low-iron, [[Toughened glass|tempered glass]]. Such glass can withstand significant hail without breaking, which is one of the reasons that flat-plate collectors are considered the most durable collector type.

Unglazed or formed collectors are similar to flat-plate collectors, except they are not thermally insulated nor physically protected by a glass panel. Consequently, these types of collectors are much less efficient when water temperature exceeds ambient air temperatures. For pool heating applications, the water to be heated is often colder than the ambient roof temperature, at which point the lack of thermal insulation allows additional heat to be drawn from the surrounding environment.<ref name="LanePt1">{{cite journal|year=2003|title=Solar pool heating basics, Part 1|journal=HomePower|volume=94|pages=70–77|author=D. Lane}}</ref>

====Evacuated tube====
[[File:Sistema por Hidroneumatico (Calentador solar para alta presión).JPG|thumb|Evacuated tube solar water heater on a roof]]
Evacuated tube collectors (ETC) are a way to reduce the heat loss,<ref name="C. Marken 2009" /> inherent in flat plates. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes.<ref>{{cite journal|year=2007|title=Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube|journal=Renewable Energy|volume=32|issue=5|pages=772|doi=10.1016/j.renene.2006.03.016|author1=Yong Kim|author2=Taebeom Seo|bibcode=2007REne...32..772K }}</ref> Since two flat glass sheets are generally not strong enough to withstand a vacuum, the vacuum is created between two concentric tubes. The inner tube is coated with a thermal absorber.<ref>{{cite journal|year=1999|title=Selective absorbing surface for evacuated solar collector tubes|journal=Renewable Energy|volume=16|issue=1–4|pages=632–634|doi=10.1016/S0960-1481(98)00240-7|author1=Shi Yueyan|author2=Yang Xiaoji|bibcode=1999REne...16..632S }}</ref> Vacuum life varies from collector to collector, from 5 years to 15 years.

In addition, evacuated tubes contain a very efficient inner system to transfer heat from the tubes to the water in the header at the top of the array. This comprises a copper tube partly filled with a volatile liquid, sealed at both ends, and running down the centre of the tube. The top end of this tube plugs into a socket in the (water filled) header. Heat absorbed by the evacuated tube causes some of the volatile liquid in the copper tube to evaporate and pass to the header. At the header the gas then condenses, releasing heat there, and the liquid runs back down inside the tube.
Since the evacuated tubes themselves do not contain water, they are not vulnerable to frosting. The water passes through the thermally insulated header only, and in climates with moderate frosts, no draindown system is required. 

Also worth noting, since ETCs are (also) exposed on the underside: where they are installed raised from the roof (as in the illustration) they will capture not only direct heat from above, but also heat radiated from surrounding roof on the underside.

Flat plate collectors are generally more efficient than ETC in full sunshine conditions. However, the energy output of flat plate collectors is reduced slightly more than ETCs in cloudy or extremely cold conditions.<ref name="C. Marken 2009" /> Most ETCs are made out of annealed glass, which is susceptible to [[hail]], failing given roughly golf ball -sized particles. ETCs made from "coke glass", which has a green tint, are stronger and less likely to lose their vacuum, but efficiency is slightly reduced due to reduced transparency. ETCs can gather energy from the sun all day long at low angles due to their tubular shape.<ref>{{cite journal|last2=Saidur|first2=R.|last3=Mekhilef|first3=Saad|last4=Mahian|first4=Omid|date=1 November 2015|title=Progress and latest developments of evacuated tube solar collectors|journal=Renewable and Sustainable Energy Reviews|volume=51|pages=1038–1054|doi=10.1016/j.rser.2015.07.016|first1=M. A.|last1=Sabiha|bibcode=2015RSERv..51.1038S }}</ref>

===Pump ===

====PV pump====
One way to power an active system is via a [[photovoltaic]] (PV) [[PV module|panel]]. To ensure proper pump performance and longevity, the (DC) pump and PV panel must be suitably matched. Although a PV-powered pump does not operate at night, the controller must ensure that the pump does not operate when the sun is out but the collector water is not hot enough.

PV pumps offer the following advantages:
* Simpler/cheaper installation and maintenance
* Excess PV output can be used for household electricity use or put back into the grid
* Can dehumidify living space<ref>{{Cite web|url=https://www.greenbuildingadvisor.com/article/getting-into-hot-water-part-1|title=Getting into Hot Water — Part 1|date=September 12, 2012|website=GreenBuildingAdvisor}}</ref>
* Can operate during a power outage
* Avoids the carbon consumption from using grid-powered pumps

====Bubble pump====
[[File:Bubble pump.ogv|thumb|The bubble separator of a bubble-pump system]]
A bubble pump (also known as geyser pump) is suitable for flat panel as well as vacuum tube systems. In a bubble pump system, the closed HTF circuit is under reduced pressure, which causes the liquid to boil at low temperature as the sun heats it. The steam bubbles form a geyser, causing an upward flow. The bubbles are separated from the hot fluid and condensed at the highest point in the circuit, after which the fluid flows downward toward the heat exchanger caused by the difference in fluid levels.<ref>A van Houten (Sunnovations), [http://www.sunnovations.com/content/how-sunnovations-system-works How a Geyser Pump works] {{webarchive|url=https://web.archive.org/web/20110114224043/http://www.sunnovations.com/content/how-sunnovations-system-works |date=2011-01-14 }}</ref><ref>Wilfried C. Sorensen (1985) [https://patents.google.com/patent/US4607688 Autogeneous solar water heater], US Patent 4607688.</ref><ref>[http://www.bubbleactionpumps.com/overview/pumpovr1.htm Bubble pump description] at bubbleactionpumps.com</ref> The HTF typically arrives at the heat exchanger at 70&nbsp;°C and returns to the circulating pump at 50&nbsp;°C. Pumping typically starts at about 50&nbsp;°C and increases as the sun rises until equilibrium is reached.{{Citation needed|date=March 2025}}

===Controller===
{{Main|Solar controller}}
A ''differential controller'' senses temperature differences between water leaving the solar collector and the water in the storage tank near the heat exchanger. The controller starts the pump when the water in the collector is sufficiently about 8–10&nbsp;°C warmer than the water in the tank, and stops it when the temperature difference reaches 3–5&nbsp;°C. This ensures that stored water always gains heat when the pump operates and prevents the pump from excessive cycling on and off. (In direct systems the pump can be triggered with a difference around 4&nbsp;°C because they have no heat exchanger.){{Citation needed|date=March 2025}}

===Tank===
The simplest collector is a water-filled metal tank in a sunny place. The sun heats the tank. This was how the first systems worked.<ref name="CSC" /> This setup would be inefficient due to the equilibrium effect: as soon as heating of the tank and water begins, the heat gained is lost to the environment and this continues until the water in the tank reaches ambient temperature. The challenge is to limit the heat loss.
* The storage tank can be situated lower than the collectors, allowing increased freedom in system design and allowing pre-existing storage tanks to be used.
* The storage tank can be hidden from view.
* The storage tank can be placed in conditioned or semi-conditioned space, reducing heat loss.
* Drainback tanks can be used.{{Citation needed|date=March 2025}}

====Insulated tank====
ICS or batch collectors reduce heat loss by thermally insulating the tank.<ref name="C. Marken 2009">{{cite journal|author=C. Marken|year=2009|title=Solar collectors: Behind the glass|journal=HomePower|volume=133|pages=70–76|url=http://www.homepower.com/articles/solar-collectors-behind-glass|access-date=2013-04-22|archive-date=2013-01-28|archive-url=https://web.archive.org/web/20130128023233/http://www.homepower.com/articles/solar-collectors-behind-glass|url-status=dead}}</ref><ref>{{cite journal|author1=C. Schmidt |author2=A. Goetzberger A. |doi=10.1016/0038-092X(90)90033-9|title=Single-tube integrated collector storage systems with transparent insulation and involute reflector|year=1990|journal=Solar Energy|volume=45|issue=2|pages=93|bibcode=1990SoEn...45...93S}}</ref> This is achieved by encasing the tank in a glass-topped box that allows heat from the sun to reach the water tank.<ref>{{cite journal|author1=M. Smyth |author2=P.C. Eames |author3=B. Norton |year=2006|title= Integrated collector storage solar water heaters|doi=10.1016/j.rser.2004.11.001|journal=Renewable and Sustainable Energy Reviews|volume=10|issue=6|pages=503|bibcode=2006RSERv..10..503S }}</ref> The other walls of the box are thermally insulated, reducing convection and radiation.<ref name=r1/> The box can also have a reflective surface on the inside. This reflects heat lost from the tank back towards the tank. In a simple way one could consider an ICS solar water heater as a water tank that has been enclosed in a type of 'oven' that retains heat from the sun as well as heat of the water in the tank. Using a box does not eliminate heat loss from the tank to the environment, but it largely reduces this loss.

Standard ICS collectors have a characteristic that strongly limits the efficiency of the collector: a small surface-to-volume ratio.<ref>{{cite journal|author1=Y. Tripanagnostopoulos |author2=M. Souliotis |author3=T. Nousia |doi=10.1016/S0960-1481(98)00248-1|title=Solar ICS systems with two cylindrical storage tanks|year=1999|journal=Renewable Energy|volume=16|issue=1–4 |pages=665–668|bibcode=1999REne...16..665T }}</ref> Since the amount of heat that a tank can absorb from the sun is largely dependent on the surface of the tank directly exposed to the sun, it follows that the surface  size defines the degree to which the water can be heated by the sun. Cylindrical objects such as the tank in an ICS collector have an inherently  small surface-to-volume ratio. Collectors attempt to increase this ratio for efficient warming of the water. Variations on this basic design include collectors that combine smaller water containers and evacuated glass tube technology, a type of ICS system known as an Evacuated Tube Batch (ETB) collector.<ref name="C. Marken 2009"/>

==Applications==
{{More citations needed section|date=March 2025}} 

===Evacuated tube===

ETSCs can be more useful than other solar collectors during winter season. ETCs can be used for heating and cooling purposes in industries like pharmaceutical and drug, paper, leather and textile and also for residential houses, hospitals, nursing home, hotels, swimming pool etc.

An ETC can operate at a range of temperatures from medium to high for solar hot water, swimming pool, air conditioning and solar cooker.

ETCs higher operational temperature range (up to {{convert|200|C|F}}) makes them suitable for industrial applications such as steam generation, heat engine and solar drying.

===Swimming pools===
[[Swimming pool#Covers|Floating pool covering systems]] and separate STCs are used for pool heating.

Pool covering systems, whether solid sheets or floating disks, act as insulation and reduce heat loss. Much heat loss occurs through evaporation, and using a cover slows evaporation.

STCs for nonpotable pool water use are often made of plastic. [[Swimming pool|Pool]] water is mildly corrosive due to chlorine. Water is circulated through the panels using the existing pool filter or supplemental pump. In mild environments, unglazed plastic collectors are more efficient as a direct system. In cold or windy environments evacuated tubes or flat plates in an indirect configuration are used in conjunction with a heat exchanger. This reduces corrosion. A fairly simple [[#Active systems with intelligent controllers|differential temperature controller]] is used to direct the water to the panels or heat exchanger either by turning a valve or operating the pump. Once the pool water has reached the required temperature, a diverter valve is used to return water directly to the pool without heating.<ref name="D. Lane 2003">{{cite journal|author=D. Lane|year=2003|title= Solar pool heating basics, Part 2|journal=HomePower |volume=95|pages=60–67}}</ref> Many systems are configured as drainback systems where the water drains into the pool when the water pump is switched off.

The collector panels are usually mounted on a nearby roof, or ground-mounted on a tilted rack. Due to the low temperature difference between the air and the water, the panels are often formed collectors or unglazed flat plate collectors. A simple rule-of-thumb for the required panel area needed is 50%  of the pool's surface area.<ref name="D. Lane 2003"/> This is for areas where pools are used in the summer season only. Adding solar collectors to a conventional outdoor pool, in a cold climate, can typically extend the pool's comfortable usage by months and more if an insulating pool cover is used.<ref name=LanePt1/> When sized at 100% [[Solar coverage rate|coverage]] most solar hot water systems are capable of heating a pool anywhere from as little as 4&nbsp;°C for a wind-exposed pool, to as much as 10&nbsp;°C for a wind-sheltered pool covered consistently with a solar pool blanket.<ref>{{Cite web|url=https://www.ecoonline.com.au/do-solar-pool-heater-work/|title=How Much Will A Solar Pool Heating System Heat My Pool}}</ref>

An [[active solar]] energy system analysis program may be used to optimize the solar pool heating system before it is built.

==Energy production==
[[File:Laundromat-SolarCell.png|thumb|A laundromat in [[California]] with panels on the roof providing hot washing water]]

The amount of heat delivered by a solar water heating system depends primarily on the amount of heat delivered by the sun at a particular place ([[insolation]]). In the tropics insolation can be relatively high, e.g. 7&nbsp;kWh/m<sup>2</sup> per day, versus e.g., 3.2&nbsp;kWh/m<sup>2</sup> per day in [[temperate]] areas. Even at the same latitude average insolation can vary a great deal from location to location due to differences in local weather patterns and the amount of overcast. Calculators are available for estimating [[insolation]] at a site.<ref>{{cite web |url=http://sunbird.jrc.it/pvgis/imaps |title=interactive maps |publisher=Sunbird.jrc.it |date=30 October 2008 |access-date=10 February 2012 |url-status=dead |archive-url=https://archive.today/20120719103622/http://sunbird.jrc.it/pvgis/imaps |archive-date=19 July 2012 }}</ref><ref>{{cite web|url=http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/ |title=A Performance Calculator for Grid-Connected PV Systems |publisher=Rredc.nrel.gov |access-date=10 February 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120118101606/http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/ |archive-date=January 18, 2012 }}</ref><ref>{{cite web|url=http://www.nrel.gov |title=National Renewable Energy Laboratory (NREL) Home Page |publisher=Nrel.gov |date=6 February 2012 |access-date=10 February 2012}}</ref>

Below is a table that gives a rough indication of the specifications and energy that could be expected from a solar water heating system involving some 2 m<sup>2</sup> of absorber area of the collector, demonstrating two evacuated tube and three flat plate solar water heating systems. Certification information or figures calculated from those data are used. The bottom two rows give estimates for daily energy production (kWh/day) for a tropical and a [[temperate]] scenario. These estimates are for heating water to 50&nbsp;°C above ambient temperature.

With most solar water heating systems, the energy output scales linearly with the collector surface area.<ref>[https://web.archive.org/web/20070220142041/http://www.solar-rating.org/certification/certification.htm SRCC Certification Programs]. solar-rating.org</ref>
{| class="wikitable"
|+ Daily energy production (kW<sub>th</sub>.h) of five solar thermal systems. The evac tube systems used below both have 20 tubes.
|-
| ''Technology''
| Flat plate
| Flat plate
| Flat plate
| ETC
| ETC
|-
| ''Configuration''
| Direct active
| Thermo{{shy}}siphon
| Indirect active
| Indirect active
| Direct active
|-
|'' Overall size (m<sup>2</sup>)''
| 2.49
| 1.98
| 1.87
| 2.85
| 2.97
|-
|''Absorber size (m<sup>2</sup>)''
| 2.21
| 1.98
| 1.72
| 2.85
| 2.96
|-
| ''Maximum efficiency''
| 0.68
| 0.74
| 0.61
| 0.57
| 0.46
|-
|  ''Energy production (kWh/day):''<br /> '' – Insolation 3.2&nbsp;kWh/m<sup>2</sup>/day ([[temperate]])''<br /> – ''e.g. Zurich, Switzerland''
| 5.3
| 3.9
| 3.3
| 4.8
| 4.0
|-
|   '' – Insolation 6.5&nbsp;kWh/m<sup>2</sup>/day (tropical)''<br /> – ''e.g. Phoenix, USA''
| 11.2
| 8.8
| 7.1
| 9.9
| 8.4
|}

The figures are fairly similar between the above collectors, yielding some 4&nbsp;kWh/day in a temperate climate and some 8&nbsp;kWh/day in a tropical climate when using a collector with a 2 m<sup>2</sup> absorber. In the [[temperate]] scenario this is sufficient to heat 200 litres of water by some 17&nbsp;°C. In the tropical scenario the equivalent heating would be by some 33&nbsp;°C. Many thermosiphon systems have comparable energy output to equivalent active systems. The efficiency of evacuated tube collectors is somewhat lower than for flat plate collectors because the absorbers are narrower than the tubes and the tubes have space between them, resulting in a significantly larger percentage of inactive overall collector area. Some methods of comparison<ref>ISO 9806-2:1995. Test methods for solar collectors – Part 2: Qualification test procedures. International Organization for Standardization, Geneva, Switzerland</ref> calculate the efficiency of evacuated tube collectors based on the actual absorber area and not on the space occupied as has been done in the above table. Efficiency is reduced at higher temperatures.

==Costs==
In sunny, warm locations, where freeze protection is not necessary, an ICS (batch type) solar water heater can be cost effective.<ref name=r1>{{cite journal|author1=M. Souliotis |author2=S. Kalogirou |author3=Y. Tripanagnostopoulos |doi=10.1016/j.renene.2008.09.007|title=Modelling of an ICS solar water heater using artificial neural networks and TRNSYS|year=2009|journal=Renewable Energy|volume=34|issue=5|pages=1333|bibcode=2009REne...34.1333S }}</ref>
In higher latitudes, design requirements for cold weather add to system complexity and cost.
This increases ''initial'' costs, but not life-cycle costs. The biggest single consideration is therefore the large initial financial outlay of solar water heating systems.<ref>{{cite journal|author=H. M. Healey|year=2007|title= Economics of Solar|journal=Cogeneration & Distributed Generation Journal|volume =22|pages=35–49|doi=10.1080/15453660709509122|issue=3|bibcode=2007CDGJ...22c..35H }}</ref> Offsetting this expense can take years.<ref name="H. Crawford 2003">{{cite journal|author1=R. H. Crawford |author2=G. J. Treloar |author3=B. D. Ilozor |author4=P. E. D. Love |doi=10.1080/09613210210160800|title=Comparative greenhouse emissions analysis of domestic solar hot water systems|year=2003|journal=Building Research & Information|volume=31|issue=1 |pages=34–47|bibcode=2003BuRI...31...34C |s2cid=111202685 }}</ref>
The payback period is longer in temperate environments.<ref name=Marken>{{cite journal|author1=C. Marken |author2=J. Sanchez |year=2008|title= PV vs. Solar Water Heating: Simple Solar Payback|journal=HomePower|volume= 127|pages=40–45|url=http://www.homepower.com/articles/pv-vs-solar-water-heating}}</ref>
Since solar energy is free, operating costs are small.
At higher latitudes, solar heaters may be less effective due to lower insolation, possibly requiring larger and/or dual-heating systems.<ref name=Marken/> In some countries government incentives can be significant.

Cost factors (positive and negative) include:
* Price of solar water heater (more complex systems are more expensive)
* Efficiency
* Installation cost
* Electricity used for pumping
* Price of water heating fuel (e.g. gas or electricity) saved per kWh
* Amount of water heating fuel used
* Initial and/or recurring government subsidy
* Maintenance cost (e.g. antifreeze or pump replacements)
* Savings in maintenance of conventional (electric/gas/oil) water heating system

Payback times can vary greatly due to regional sun, extra cost due to frost protection needs of collectors, household hot water use etc. For instance in central and southern Florida the payback period could easily be 7 years or less rather than the 12.6 years indicated on the chart for the United States.<ref>[http://www.fsec.ucf.edu/en/consumer/solar_hot_water/homes/calculator/index.htm Simplified Residential Solar Hot Water System Calculator], Florida Solar Energy Center (2007).</ref>

{| class="wikitable" | Solar Water Heating: Costs and Payback periods
|+ Costs and payback periods for residential SWH systems with savings of 200&nbsp;kWh/month (using 2010 data), ex maintenance costs, subsidies and installation costs
|-
! Country
! Curr{{shy}}ency
! System cost
! Subsidy(%)
! Effective cost
! Electricity cost/kWh
! Electricity savings/month
! Payback period(y)
|-
| style="text-align:left;"|{{BRA}}|| style="text-align:left;"|BRL || style="text-align:right;"| 2500<ref>Milton S. & Kaufman S. (2005). [http://www.green-markets.org/Downloads/SWH_carbon.pdf Solar Water Heating as a Climate Protection Strategy: The Role for Carbon Finance]. Green Markets International. Arlington MA, US</ref> || style="text-align:right;"|0 || style="text-align:right;"| 2500 || style="text-align:right;"| 0.25 || style="text-align:right;"| 50 || style="text-align:right;"| 4.2
|-
| style="text-align:left;"|{{RSA}}|| style="text-align:left;"|ZAR || style="text-align:right;"| 14000 || style="text-align:right;"|15<ref>{{cite web|url=http://www.eskom.co.za |title=Eskom |publisher=Eskom |access-date=10 February 2012}}</ref> || style="text-align:right;"| 11900 || style="text-align:right;"| 0.9 || style="text-align:right;"| 180 || style="text-align:right;"| 5.5
|-
| style="text-align:left;"|{{AUS}}|| style="text-align:left;"|AUD || style="text-align:right;"| 5000<ref>{{cite web |url=http://www.enviro-friendly.com/evacuated-tube-solar-hot-water.shtml |title=Hills Solar Evacuated Tube Solar Hot Water Systems |publisher=Enviro-friendly.com |access-date=10 February 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120217122215/http://www.enviro-friendly.com/evacuated-tube-solar-hot-water.shtml |archive-date=17 February 2012 }}</ref> || style="text-align:right;"|40<ref name="environment.gov.au">[https://web.archive.org/web/20090206120159/http://environment.gov.au/energyefficiency/index.html Energy Efficient Homes Package]. environment.gov.au</ref> || style="text-align:right;"| 3000 || style="text-align:right;"| 0.18<ref>{{cite web|url=http://www.aer.gov.au/content/index.phtml/itemId/717887 |title=AER issues report on high electricity prices in South Australia |publisher=Aer.gov.au |date=4 March 2008 |access-date=10 February 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120303133939/http://www.aer.gov.au/content/index.phtml/itemId/717887 |archive-date=March 3, 2012 }}</ref> || style="text-align:right;"| 36 || style="text-align:right;"| 6.9
|-
| style="text-align:left;"|{{BEL}}|| style="text-align:left;"|EUR || style="text-align:right;"| 4000<ref>[http://www2.vlaanderen.be/economie/energiesparen/doc/folder_zonneboiler.pdf WAT kost een zonneboiler?] {{webarchive|url=https://web.archive.org/web/20091104181439/http://www2.vlaanderen.be/economie/energiesparen/doc/folder_zonneboiler.pdf |date=2009-11-04 }} vlaanderen.be, 30 April 2008.</ref> || style="text-align:right;"|50<ref>{{cite web |url=http://www.vlaanderen.be/servlet/Satellite?pagename=Infolijn/View&cid=1090509343446&c=Solution_C&p=1186804409590&context= |title=Premies voor energiebesparende maatregelen {{pipe}} Vlaanderen.be: uw link met de overheid |publisher=Vlaanderen.be |access-date=10 February 2012 |url-status=dead |archive-url=https://web.archive.org/web/20110927090915/http://www.vlaanderen.be/servlet/Satellite?pagename=Infolijn%2FView&cid=1090509343446&c=Solution_C&p=1186804409590&context= |archive-date=September 27, 2011 }}</ref> || style="text-align:right;"| 2000 || style="text-align:right;"| 0.1<ref>{{cite web|url=http://www.electrabel.be/residential/energy_saving/duplicate/build_renovate/build_renew_hot_water_nl.aspx |title=No aspx {{pipe}} Electrabel |publisher=Electrabel.be |access-date=10 February 2012}}</ref> || style="text-align:right;"| 20 || style="text-align:right;"| 8.3
|-
| style="text-align:left;"|{{USA}}|| style="text-align:left;"|USD || style="text-align:right;"| 5000<ref name="srpnet.com">{{cite web|url=http://www.srpnet.com/environment/earthwise/solar/default.aspx |title=SRP EarthWise Solar Energy for your home |publisher=Srpnet.com |access-date=10 February 2012}}</ref> || style="text-align:right;"|30<ref>{{cite web|url=http://www.energystar.gov/index.cfm?c=tax_credits.tx_index |title=Federal Tax Credits for Energy Efficiency : ENERGY STAR |publisher=Energystar.gov |date=2012-01-03 |access-date=2012-06-23}}</ref> || style="text-align:right;"| 3500 || style="text-align:right;"| 0.1158<ref>{{cite web| url=http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_b.html| title=Average Retail Price of Electricity to Ultimate Customers by End-Use Sector, by State}}</ref> || style="text-align:right;"| 23.16 || style="text-align:right;"| 12.6
|-
| style="text-align:left;"|{{UK}}|| style="text-align:left;"|GBP || style="text-align:right;"| 4800<ref>{{cite web|url=http://www.energysavingtrust.org.uk/Generate-your-own-energy/Solar-water-heating |title=Solar water heating systems explained – benefits, costs, savings, earnings, suitability |publisher=Energysavingtrust.org.uk |access-date=2012-06-23}}</ref> || style="text-align:right;"|0 || style="text-align:right;"| 4800 || style="text-align:right;"| 0.11<ref>{{cite web|url=http://www.ukpower.co.uk/tools/running_costs_electricity/ |title=Electricity Running Cost Calculator {{pipe}} Electricity Prices {{pipe}} Electricity Costs |publisher=Ukpower.co.uk |access-date=2012-06-23}}</ref> || style="text-align:right;"| 22 || style="text-align:right;"| 18.2
|}
{{clear}}

The payback period is shorter given greater insolation. However, even in temperate areas, solar water heating is cost effective. The payback period for photovoltaic systems has historically been much longer.<ref name=Marken/> Costs and payback period are shorter if no complementary/backup system is required.<ref name="H. Crawford 2003"/> thus extending the payback period of such a system.

===Subsidies===
Australia operates a system of Renewable Energy Credits, based on national renewable energy targets.<ref name="environment.gov.au" />

The [[Toronto Solar Neighbourhoods Initiative]] offers subsidies for the purchase of solar water heating units.<ref>{{Cite web|url=https://www.toronto.com/news/|archiveurl=https://web.archive.org/web/20110719115247/http://www.insidetoronto.com/Article/50574|url-status=dead|title=Breaking News |archivedate=July 19, 2011|website=Toronto.com}}</ref>

==Energy footprint and life cycle assessment==
{{More citations needed section|date=March 2025}} 

===Energy footprint===
The source of electricity in an active SWH system determines the extent to which a system contributes to atmospheric carbon during operation. Active solar thermal systems that use mains electricity to pump the fluid through the panels are called 'low carbon solar'. In most systems the pumping reduces the energy savings by about 8% and the carbon savings of the solar by about 20%.<ref>C. Martin and M. Watson (2001). DTI publication URN 01/1292. London, UK</ref> However, low power pumps operate with 1-20W.<ref>{{cite web|url=http://lainginc.itt.com/LG-pump-DC-Solar-Pumps.asp |title=DC Solar Pumps |work=lainginc.itt.com |access-date=5 November 2010 |url-status=unfit |archive-url=https://web.archive.org/web/20100119014925/http://lainginc.itt.com/LG-pump-DC-Solar-Pumps.asp |archive-date=January 19, 2010 }}</ref><ref>{{cite web
 |url= http://www.bouwwereld.nl/web/Actueel/Nieuws/Nieuws/08820/Nominaties-VSK-Awards.htm
 |title=Nominaties VSK Awards
 |language=nl
 |trans-title=Laing ITT Ecocirc pump nominated for prestigious VSK award in heating category
 |work=bouwwereld.nl
 |access-date=5 November 2010
|date=2009-11-25
 }}</ref> Assuming a solar collector panel delivering 4&nbsp;kWh/day and a pump running intermittently from mains electricity for a total of 6 hours during a 12-hour sunny day, the potentially negative effect of such a pump can be reduced to about 3% of the heat produced.

However, PV-powered active solar thermal systems typically use a 5–30 W PV panel and a small, low power [[diaphragm pump]] or [[centrifugal pump]] to circulate the water. This reduces the operational carbon and energy footprint.

Alternative non-electrical pumping systems may employ thermal expansion and phase changes of liquids and gases.

===Life cycle energy assessment===
Recognised standards can be used to deliver robust and quantitative [[life cycle assessment]]s (LCA). LCA considers the financial and environmental costs of acquisition of raw materials, manufacturing, transport, using, servicing and disposal of the equipment. Elements include:
* Financial costs and gains
* Energy consumption
* CO<sub>2</sub> and other emissions
In terms of energy consumption, some 60% goes into the tank, with 30% towards the collector<ref name="G. Tsilingiridis, G 2004">{{cite journal|author1=G. Tsilingiridis, G. Martinopoulos  |author2=N. Kyriakis |name-list-style=amp |doi=10.1016/j.renene.2003.12.007|title=Life cycle environmental impact of a thermosyphonic domestic solar hot water system in comparison with electrical and gas water heating|year=2004|journal=Renewable Energy|volume=29|issue=8|pages=1277 |bibcode=2004REne...29.1277T }}</ref> (thermosiphon flat plate in this case). In Italy,<ref name="F. Ardente, G. Beccali 2005">{{cite journal|author1=F. Ardente |author2=G. Beccali |author3=M. Cellura |doi=10.1016/j.renene.2004.05.006|title=Life cycle assessment of a solar thermal collector: Sensitivity analysis, energy and environmental balances|year=2005|journal=Renewable Energy|volume=30|issue=2|pages=109|bibcode=2005REne...30..109A }}</ref> some 11 giga-joules of electricity are used in producing SWH equipment, with about 35% goes toward the tank, with another 35% towards the collector. The main energy-related impact is emissions. The energy used in manufacturing is recovered within the first 2–3 years of use (in southern Europe).

By contrast the energy payback time in the UK is reported as only 2 years. This figure was for a direct system, retrofitted to an existing water store, PV pumped, freeze tolerant and of 2.8 sqm aperture. For comparison, a PV installation took around 5 years to reach energy payback, according to the same comparative study.<ref>{{cite journal|year=2008|title=Integrated appraisal of micro-generators: Methods and applications|journal=Proceedings of the Institution of Civil Engineers - Energy|volume=161|issue=2|page=5, Fig. 1|doi=10.1680/ener.2008.161.2.73|author=S.R. Allen, G.P. Hammond, H. Harajli1, C.I. Jones, M.C. McManus and A.B. Winnett|bibcode=2008ICEE..161...73A |citeseerx=10.1.1.669.9412|s2cid=110151825 }}</ref>

In terms of CO<sub>2</sub> emissions, a large fraction of the emissions saved is dependent on the degree to which gas or electricity is used to supplement the sun. Using the Eco-indicator 99 points system as a yardstick (i.e. the yearly environmental load of an average European inhabitant) in Greece,<ref name="G. Tsilingiridis, G 2004" /> a purely gas-driven system may have fewer emissions than a solar system. This calculation assumes that the solar system produces about half of the hot water requirements of a household. But because methane (CH<sub>4</sub>) emissions from the natural gas fuel cycle<ref>"Howarth methane Biogeo lecture 2019" https://www.eeb.cornell.edu/howarth/documents/Howarth_methane-Biogeo-lecture_2019-0301.pdf</ref> dwarf the greenhouse impact of CO<sub>2</sub>, the net [[greenhouse gas emissions|greenhouse emissions]] (CO<sub>2</sub>e) from gas-driven systems are vastly greater than for solar heaters, especially if supplemental electricity is also from carbon-free generation.{{citation needed|date=August 2019}}

A test system in Italy produced about 700&nbsp;kg of CO<sub>2</sub>, considering all the components of manufacture, use and disposal. Maintenance was identified as an emissions-costly activity when the heat transfer fluid (glycol-based) was replaced. However, the emissions cost was recovered within about two years of use of the equipment.<ref name="F. Ardente, G. Beccali 2005" />

In Australia, life cycle emissions were also recovered. The tested SWH system had about 20% of the impact of an electrical water heater and half that of a gas water heater.<ref name="H. Crawford 2003" />

Analysing their lower impact retrofit freeze-tolerant solar water heating system, Allen ''et al.'' (qv) reported a production CO<sub>2</sub> impact of 337&nbsp;kg, which is around half the environmental impact reported in the Ardente ''et al.'' (qv) study.

==System specification and installation==
{{More citations needed section|date=March 2025}}
* Most SWH installations require backup heating.
* The amount of hot water consumed each day must be replaced and heated. In a solar-only system, consuming a high fraction of the water in the reservoir implies significant reservoir temperature variations. The larger the reservoir the smaller the daily temperature variation.
* SWH systems offer significant [[Economies of scale|scale economies]] in collector and tank costs.<ref name="G. Tsilingiridis, G 2004"/> Thus the most economically efficient scale meets 100% of the heating needs of the application.
* Direct systems (and some indirect systems using heat exchangers) can be retrofitted to existing stores.
* Equipment components must be insulated to achieve full system benefits. The installation of efficient insulation significantly reduces heat loss.
* The most efficient PV pumps start slowly in low light levels, so they may cause a small amount of unwanted circulation while the collector is cold. The controller must prevent stored hot water from this cooling effect.
* Evacuated tube collector arrays can be adjusted by removing/adding tubes or their heat pipes, allowing customization during/after installation.
* Above 45 degrees latitude, roof mounted sun-facing collectors tend to outproduce wall-mounted collectors. However, arrays of wall-mounted steep collectors can sometimes produce more useful energy because gains in used energy in winter can offset the loss of unused (excess) energy in summer.

==Standards==

===Europe===

*EN 806: Specifications for installations inside buildings conveying water for human consumption. General.
*EN 1717: Protection against pollution of potable water in water installations and general requirements of devices to prevent pollution by backflow.
* EN 60335: Specification for safety of household and similar electrical appliances. (2–21)
* UNE 94002:2005 Thermal solar systems for domestic hot water production. Calculation method for heat demand.

===United States===

*OG-300: OG-300 Certification of Solar Water Heating Systems.<ref>{{cite web |url= http://www.solar-rating.org/certification/system.html |title=Solar Rating & Certification Corporation – System Ratings |work=solar-rating.org |year=2016 |access-date=June 23, 2016}}</ref>

===Canada===

*[http://shop.csa.ca/en/canada/renewable-thermal-energy/cancsa-f378-series-11/invt/27008102011 CAN/CSA-F378 Series 11 (Solar collectors)]
*[http://shop.csa.ca/en/canada/renewable-thermal-energy/cancsa-f379-series-09-r2013/invt/27029412009 CAN/CSA-F379 Series 09 (Packaged solar domestic hot water systems)]
*[http://www.solar-rating.org/standards/SRCC_Standard_600_For_Solar_Concentrating_Collectors.pdf SRCC Standard 600 (Minimum standard for solar thermal concentrating collectors)]

===Australia===

*Renewable Energy (Electricity) Act 2000
*Renewable Energy (Electricity) (Large-scale Generation Shortfall Charge) Act 2000
*Renewable Energy (Electricity) (Small-scale Technology Shortfall Charge) Act 2010
*Renewable Energy (Electricity) Regulations 2001
*Renewable Energy (Electricity) Regulations 2001 - STC Calculation Methodology for Solar Water Heaters and [[air source heat pump]] Water Heaters
*Renewable Energy (Electricity) Amendment (Transitional Provision) Regulations 2010
*Renewable Energy (Electricity) Amendment (Transitional Provisions) Regulations 2009

All relevant participants of the Large-scale Renewable Energy Target and Small-scale Renewable Energy Scheme must comply with the above Acts.<ref name=CER2013>{{cite web|title=RET Compliance|url=http://ret.cleanenergyregulator.gov.au/For-Industry/ret-compliance|publisher=Australian Government, Clean Energy Regulator|access-date=2014-09-25 |date=2 January 2013}}</ref>

==Worldwide use==
[[File:South Africa-Kouga-Solar hot water-001.jpg|thumb|Solar hot water system installed on low cost housing in the [[Kouga Local Municipality]], South Africa]]

{| class="wikitable sortable" | Solar Heating Capacity 2005–2010 (GWth)
|+ Top countries using solar thermal power, worldwide (GW<sub>th</sub>)<ref name="REN21 2009"/><ref>[https://web.archive.org/web/20100307001055/http://ren21.net/pdf/RE_GSR_2009_Update.pdf Renewables Global Status Report 2009 Update]. Deutsche Gesellschaft für Technische Zusammenarbeit. ren21.net</ref><ref>{{cite web|url=http://www.ren21.net/globalstatusreport/REN21_GSR_2010_full.pdf |title=Renewables Global Status Report 2010 |publisher=REN21 |access-date=2012-06-23 |url-status=dead |archive-url=https://web.archive.org/web/20100820221548/http://www.ren21.net/globalstatusreport/REN21_GSR_2010_full.pdf |archive-date=August 20, 2010 }}</ref><ref name="baro"/><ref>{{cite web|url=http://www.iea-shc.org/publications/downloads/Solar_Heat_Worldwide-2011.pdf |title=Solar Heat Worldwide |author1=Werner Weiss  |author2=Franz Mauthner  |name-list-style=amp |date=May 2011 |access-date=2012-06-23 |url-status=dead |archive-url=https://web.archive.org/web/20110812220754/http://www.iea-shc.org/publications/downloads/Solar_Heat_Worldwide-2011.pdf |archive-date=August 12, 2011 }}</ref><ref>Werner Weiss and Franz Mauthner [https://web.archive.org/web/20120614195029/http://www.iea-shc.org/publications/downloads/Solar_Heat_Worldwide-2012.pdf Solar Heat Worldwide Markets and Contribution to the Energy Supply 2010].  iea-shc.org</ref><ref name=bar11>[http://www.eurobserv-er.org/pdf/baro209-ST_H.pdf Solar thermal and concentrated solar power barometer]. EurObserv'ER n° 209 (May 2012).</ref>
|-
! #
! Country
! 2005
! 2006
! 2007
! 2008
! 2009
! 2010
! 2011
! 2012
! 2013<ref name=iea2015>{{cite web|last1=Mauthner|first1=Franz|last2=Weiss|first2=Werner|last3=Spörk-Dür|first3=Monika|title=Solar Heat Worldwide|url=https://www.iea-shc.org/data/sites/1/publications/Solar-Heat-Worldwide-2015.pdf|publisher=International Energy Agency Solar Heating & Cooling Programme|access-date=6 April 2017|date=June 2015}}</ref>
!2014
!2015
!2016
!2017
!2018
!2019<ref name=iea2019>{{cite web|last1=Mauthner|first1=Franz|last2=Spörk-Dür|first2=Monika|title=Solar Heat Worldwide|url=https://www.iea-shc.org/data/sites/1/publications/Solar-Heat-Worldwide-2021.pdf|publisher=International Energy Agency Solar Heating & Cooling Programme|access-date=19 January 2025|date=May 2021}}</ref>
!2020<ref name=iea2020>{{cite web|last1=Mauthner|first1=Franz|last2=Spörk-Dür|first2=Monika|title=Solar Heat Worldwide|url=https://www.iea-shc.org/data/sites/1/publications/Solar-Heat-Worldwide-2022.pdf|publisher=International Energy Agency Solar Heating & Cooling Programme|access-date=19 January 2025|date=May 2022}}</ref>
!2021<ref name=iea2021>{{cite web|last1=Mauthner|first1=Franz|last2=Spörk-Dür|first2=Monika|title=Solar Heat Worldwide|url=https://www.iea-shc.org/data/sites/1/publications/Solar-Heat-Worldwide-2023.pdf|publisher=International Energy Agency Solar Heating & Cooling Programme|access-date=19 January 2025|date=May 2023}}</ref>
!2022<ref name=iea2022>{{cite web|last1=Mauthner|first1=Franz|last2=Spörk-Dür|first2=Monika|title=Solar Heat Worldwide|url=https://www.iea-shc.org/data/sites/1/publications/Solar-Heat-Worldwide-2024.pdf|publisher=International Energy Agency Solar Heating & Cooling Programme|access-date=19 January 2025|date=May 2024}}</ref>
|- style="text-align:right;"
| 1 || style="text-align:left;"| {{Flagu|China}} || 55.5 || 67.9 || 84.0 || 105.0 || 101.5 || 117.6 || - || - || 262.3 || 289.5 || 309.5 || 324.5 || 334.5 || 337.6 || 346.5 || 364.0 || 381.6 || 396.4
|- style="text-align:right;"
| – || style="text-align:left;"| ''{{Flagu|EU}}'' || 11.2 || 13.5 || 15.5 || 20.0 || 22.8 || 23.5 || 25.6 || 29.7 || 31.4 || 47.5 || 49.2 || 51.8 || 54.3 || 56.8 || 58.9 || 59.5 || 62.2 || 63.2
|- style="text-align:right;"
| 2 || style="text-align:left;"| {{Flagu|Turkey}} || 5.7 || 6.6 || 7.1 || 7.5 || 8.4 || 9.3 || - || - || 11.0 || 12.7 || 13.6 || 14.9 || 16.3 || 17.6 || 18.1 || 18.4 || 19.0 || 19.2
|- style="text-align:right;"
| 3 || style="text-align:left;"| {{Flagu|United States}} || 1.6 || 1.8 || 1.7 || 2.0 || 14.4 || 15.3 || - || - || 16.8 || 17.1 || 17.4 || 17.7 || 17.9 || 18.0 || 18.2 || 18.2 || 18.3 || 18.4
|- style="text-align:right;"
| 4 || style="text-align:left;"| {{Flagu|Germany}} || – || – || – || 7.8 || 8.9 || 9.8 || 10.5 || 11.4 || 12.1 || 12.8 || 13.2 || 13.5 || 13.7 || 13.9 || 13.9 || 14.0 || 15.6 || 15.8
|- style="text-align:right;"
| 5 || style="text-align:left;"| {{Flagu|Brazil}} || 1.6 || 2.2 || 2.5 || 2.4 || 3.7 || 4.3 || - || - || 6.7 || 7.7 || 8.7 || 9.6 || 10.4 || 11.3 || 12.2 || 13.1 || 14.3 || 15.5
|- style="text-align:right;"
| 6 || style="text-align:left;"| {{Flagu|India}} || 1.1 || 1.2 || 1.5 || 1.7 || 2.2 || 2.8 || 3.4 || 4.5 || 4.4 || 5.2 || 6.2 || 6.7 || 8.0 || 9.5 || 10.4 || 11.5 || 12.8 || 13.5
|- style="text-align:right;"
| 7 || style="text-align:left;"| {{Flagu|Australia}} || 1.2 || 1.3 || 1.2 || 1.3 || 5.0 || 5.8 || - || - || 5.8 || 6.1 || 6.2 || 6.4 || 6.6 || 6.7 || 6.8 || 6.8 || 6.9 || 6.7
|- style="text-align:right;"
| 8 || style="text-align:left;"| {{Flagu|Mexico}} || – || – || – || - || - || - || 1.2 || 1.4 || 1.8 || 2.0 || 2.8 || 2.4 || 2.7 || 3.0 || 3.3 || 3.6 || 3.9 || 4.2
|- style="text-align:right;"
| 9 || style="text-align:left;"| {{Flagu|Greece}} || – || – || – || 2.7 || 2.9 || 2.9 || 2.9 || 2.9 || 2.9 || 3.0 || 3.1 || 3.1 || 3.2 || 3.3 || 3.4 || 3.5 || 3.6 || 3.8
|- style="text-align:right;"
| 10 || style="text-align:left;"| {{Flagu|Israel}} || 3.3 || 3.8 || 3.5 || 2.6 || 2.8 || 2.9 || - || - || 2.9 || 3.2 || 3.2 || 3.2 || 3.3 || 3.3 || 3.4 || 3.4 || 3.5 || 3.6

|-  style="text-align:right;"
| || style="text-align:left;"| ''' World''' || '''88''' || '''105''' || '''126''' || '''149''' || '''172''' || '''196''' || - || - || '''374.7''' || '''410.2''' || '''435.9''' || '''457''' || '''473.5''' || '''483''' || '''479''' || '''500''' || '''524''' || '''542.7'''
|}
{{clear}}

===European Union===
{{update section|date=May 2019}}
{| class="wikitable"
|+ Solar thermal heating in European Union (MW<sub>th</sub>)<ref>[http://www.estif.org/fileadmin/estif/content/market_data/downloads/2010%20European%20Solar%20Thermal%20Markets.pdf Solar thermal market in Europe 2010] Trends and Market Statistics, ESTIF 6/2011</ref><ref>[http://www.estif.org/fileadmin/estif/content/market_data/downloads/2009%20solar_thermal_markets.pdf Solar thermal market grows strongly in Europe 2009] ESTIF 2010</ref><ref>[http://www.estif.org/fileadmin/estif/content/market_data/downloads/2008%20Solar_Thermal_Markets_in_Europe_2008.pdf Solar thermal market grows strongly in Europe 2008] ESTIF 5/2009</ref>
|-
! #
! Country
! 2008
! 2009
! 2010<ref name="baro">[http://www.eurobserv-er.org/pdf/baro197.pdf Solar thermal energy barometer 2010] EurObserv'ER Systèmes solaires Le journal des énergies renouvelables n° 197, 5/2010</ref>
! 2011
! 2012
! 2013
|- style="text-align:right;"
| 1|| style="text-align:left;"| {{Flagu|Germany}}  || 7,766 || 9,036 || 9,831 || 10,496 || 11,416 || 12,055
|- style="text-align:right;"
| 2 || style="text-align:left;"| {{Flagu|Austria}}  || 2,268 || 3,031 || 3,227 || 2,792 || 3,448 || 3,538
|- style="text-align:right;"
| 3 || style="text-align:left;"| {{Flagu|Greece}} || 2,708 || 2,853 || 2,855 || 2,861 || 2,885 || 2,915
|- style="text-align:right;"
| 4|| style="text-align:left;"|{{Flagu|Italy}} || 1,124 || 1,410 || 1,753 || 2,152 || 2,380 || 2,590
|- style="text-align:right;"
| 5 || style="text-align:left;"| {{Flagu|Spain}} || 988|| 1,306 || 1,543 || 1,659 || 2,075 || 2,238
|- style="text-align:right;"
| 6 || style="text-align:left;"| {{Flagu|France}}  || 1,137|| 1,287 || 1,470 || 1,277 || 1,691 || 1,802
|- style="text-align:right;"
| 7 || style="text-align:left;"| {{Flagu|Poland}} || 254|| 357 || 459 || 637 || 848 || 1,040
|- style="text-align:right;"
| 8 || style="text-align:left;"| {{Flagu|Portugal}}|| 223|| 395 || 526 || 547 || 677 || 717
|- style="text-align:right;"
| 9 || style="text-align:left;"| {{Flagu|Czech Republic}}|| 116 || 148 || 216 || 265 || 625 || 681
|- style="text-align:right;"
| 10|| style="text-align:left;"| {{Flagu|Switzerland}} || 416|| 538 || 627|| -|| -|| -
|- style="text-align:right;"
| 11 || style="text-align:left;"| {{Flagu|Netherlands}} || 254|| 285 || 313 || 332 || 605 || 616
|- style="text-align:right;"
| 12 || style="text-align:left;"| {{Flagu|Denmark}} || 293|| 339 || 379 || 409 || 499 || 550
|- style="text-align:right;"
| 13 || style="text-align:left;"| {{Flagu|Cyprus}} || 485 || 490 || 491 || 499 || 486 || 476
|- style="text-align:right;"
| 14 || style="text-align:left;"| {{Flagu|UK}} || style="text-align:right;"| 270 || 333 || 374 || 460 || 455 || 475
|- style="text-align:right;"
| 15 || style="text-align:left;"| {{Flagu|Belgium}} || 188 || 204 || 230 || 226 || 334 || 374
|- style="text-align:right;"
| 16 || style="text-align:left;"| {{Flagu|Sweden}}|| 202 || 217 || 227 || 236 || 337 || 342
|- style="text-align:right;"
| 17 || style="text-align:left;"| {{Flagu|Ireland}} || 52 || 85 || 106 || 111 || 177 || 196
|- style="text-align:right;"
| 18|| style="text-align:left;"| {{Flagu|Slovenia}} || 96 || 111 || 116 || 123 || 142 || 148
|- style="text-align:right;"
| 19 || style="text-align:left;"| {{Flagu|Hungary}} || 18 || 59 || 105 || 120 || 125 || 137
|- style="text-align:right;"
| 20 || style="text-align:left;"| {{Flagu|Slovakia}} || 67 || 73 || 84 || 100 || 108 || 113
|- style="text-align:right;"
| 21 || style="text-align:left;"| {{Flagu|Romania}} * || 66 || 80 || 73 || 74 || 93 || 110
|- style="text-align:right;"
| 22 || style="text-align:left;"| {{Flagu|Bulgaria}} * || 22 || 56 || 74 || 81 || 58 || 59
|- style="text-align:right;"
| 23 || style="text-align:left;"| {{Flagu|Malta}}* || 25 || 29 || 32 || 36 || 34 || 35
|- style="text-align:right;"
| 24 || style="text-align:left;"| {{Flagu|Finland}} * || 18 || 20 || 23 || 23 || 30 || 33
|- style="text-align:right;"
| 25 || style="text-align:left;"| {{Flagu|Luxembourg}} * || 16 || 19 || 22 || 25 || 23 || 27
|- style="text-align:right;"
| 26 || style="text-align:left;"| {{Flagu|Estonia}}* || 1 || 1 || 1 || 3 || 10 || 12
|- style="text-align:right;"
| 27 || style="text-align:left;"| {{Flagu|Latvia}} * || 1 || 1 || 1 || 3 || 10 || 12
|- style="text-align:right;"
| 28 || style="text-align:left;"| {{Flagu|Lithuania}} * || 1 || 2 || 2 || 3 || 6 || 8
|- style="text-align:right;"
| Total || style="text-align:left;"| EU27+Sw (GW<sub>th</sub>) || 19,08|| 21,60|| 23.49|| 25.55|| 29.66 || 31.39
|-
| style="text-align:left;" colspan="8"|* = estimation, <sup>F</sup> = France as a whole
|}
{{clear}}

==See also==
{{Portal|Renewable energy|Energy}}

{{Commons category}}
* [[Concentrating solar power]]
* [[Passive solar]]
* [[Renewable energy commercialization]]
* [[Renewable heat]]
* [[Renewable Energy Certificate (United States)]]
* [[Solar air conditioning]]
* [[Solar air heat]]ing
* [[Solar combisystem]]
* [[Solar energy]]
* [[Solar hot water in Australia]]
* [[Solar thermal collector]]
* [[Solar thermal energy]]
* [[Solar water disinfection]]
* [[Sustainable design]]

==References==
{{Reflist}}

==External links==
*[http://www.nrel.gov/docs/fy04osti/34279.pdf Parts of a solar heating system]

{{Solar energy}}
{{Renewable energy by country}}
{{Authority control}}

{{DEFAULTSORT:Solar Water Heating}}
[[Category:Residential heating appliances]]
[[Category:Heating, ventilation, and air conditioning]]
[[Category:Solar thermal energy]]
[[Category:Sustainable energy]]
[[Category:Sustainable building]]
[[Category:Renewable energy]]
[[Category:Articles containing video clips]]