Solar hot water is the most accessible and practical of all the solar technologies

An estimated one million residential and 200,000 commercial solar water-heating systems have been installed in the United States. Although there are a large number of different types of solar water-heating systems, the basic technology is very simple. Sunlight strikes and heats an "absorber" surface within a "solar collector" or an actual storage tank. Either a heat-transfer fluid or the actual potable water to be used flows through tubes attached to the absorber and picks up the heat from it. (Systems with a separate heat-transfer-fluid loop include a heat exchanger that then heats the potable water.) The heated water is stored in a separate preheat tank or a conventional water heater tank until needed.

If additional heat is needed, it is provided by electricity or fossil-fuel energy by the conventional water-heating system. By reducing the amount of heat that must be provided by conventional water-heating, solar water-heating systems directly substitute renewable energy for conventional energy, reducing the use of electricity or fossil fuels by as much as 80%.