Supplies of solar energy are virtually unlimited; solar thermal transfer systems use some of it to heat water.

Thermal collectors are mounted on buildings, where they trap the sun's rays and use them to warm the fluid that circulates between the collectors and the solar storage cylinders. When this fluid is warm enough, a solar station activates a pump. This sends the warm fluid through the solar storage cylinders, where a heat exchanger heats the drinking water (and heating water too, in some systems) it contains.