Water Heating
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Solar hot water systems use sunlight to heat water. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water technologies have yet to show the sustained momentum they had until the 1920s.

Recent price spikes, erratic availability of conventional fuels, and other factors are renewing interest in solar heating technologies.[34][35] Approximately 14 percent (15 EJ) of the total energy used in the United States is for water heating.[36] In many climates, a solar heating system can provide 50 to 75 percent of domestic hot water use.

As of 2006, the total installed capacity of solar hot water systems is 104 GWth and growth is 15-20 percent per year.[37] China is the world leader in the deployment of solar hot water systems with 80% of the market.[37] Israel is the per capita leader in the use of solar hot water with 90 percent of homes using this technology.[38] In the United States heating swimming pools is the most successful application of solar hot water.

Solar 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.[39] 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