This description is for solar water heaters, which are an energy-efficient and cost-effective way to heat your home's water supply. Solar water heaters use the sun's energy to heat water, which is then stored in a tank for use in your home. These systems are easy
From Wikipedia
Solar water heating (SWH) is 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.