Solar water heating or solar hot water is water heated by the use of solar energy. Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage. The system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The systems may be used to heat water for a wide variety of uses, including home, business and industrial uses. Heating swimming pools, underfloor heating or energy input for space heating or cooling are more specific examples.
In many climates, a solar heating 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 combisystems) are used to provide 15 to 25% of home heating energy.
In the southern regions of Africa like Zimbabwe, solar water heaters have been gaining popularity, thanks to the Austrian-and other EU-funded projects that are promoting more environmentally friendly water heating solutions.
Residential solar thermal installations can be subdivided into two kinds of systems: compact and 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 such as 50 °C. Hence, hot water is always available. The combination of solar water heating and using the back-up heat from a wood stove chimney to heat water 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.
Among pumped options, there is an important distinction to be made regarding the sustainability of the design of the system. This relates to what source of energy powers the pump and its controls. The type of pumped solar thermal systems which use mains electricity to pump the fluid through the panels are called low carbon solar because the pumping negates the carbon savings of the solar by about 20%, according to data in a report called “Side by side testing of eight solar water heatings” by DTI UK. However, zero-carbon pumped solar thermal systems use solar electricity which is generated onsite using photovoltaics to pump the fluid and to operate its control electronics. This represents a zero operational carbon footprint and is becoming an important design goal for innovative solar thermal systems.