Solar hot water systems use sunlight to heat water. Commercial solar powered water heaters began appearing in the United States in the 1890s. Recent price spikes, erratic availability of conventional fuels, and other factors are renewing interest in solar heating technologies.

China is the world leader in the deployment of solar water heating systems with 80% of the market. Israel is the per capita leader in the use of solar hot water with 90 percent of homes using this technology. In the United States heating swimming pools is the most successful application of solar hot water.

Solar energy water heating technologies have high efficiencies relative to other solar technologies. Performance will depend upon the site of deployment, but flat-plate and evacuated-tube collectors can be expected to have efficiencies above 60 percent during normal operating conditions. In addition, solar water heating is particularly appropriate for low-temperature (25-70 °C) applications such as swimming pools, domestic hot water, and space heating. The most common types of solar water heaters are batch systems, flat plate collectors and evacuated tube collectors.

Solar hot water refers to water heated by 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, and a reservoir or tank for heat storage and subsequent use. The systems may be used to heat water for home or business use, for swimming pools, or as an energy input for space heating and cooling and industrial applications.

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 hot water heating and using the heat from a wood stove chimney to heat water can enable a hot water system to work year round in northern climates without the supplemental heat requirement of a solar hot water system being met with fossil fuels or electricity.

Solar hot water systems can be classified in different ways:

• The type of collector used.
• The location of the collector - roof mount, ground mount, wall mount.
• The location of the storage tank in relation to the collector.
• The requirement for a pump - active vs passive.
• The method of heat transfer - open-loop or closed-loop (via heat exchanger). [Wikipedia]

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