The intense energy of the sun has long been used to heat liquids. Among the first mechanical uses of the sun was a 20-square-meter, parabolic concentrating reflector that boiled water and produced steam. This steam was used in a steam-driven printing press at the 1878 World's Fair in Paris.

In the late 1800s, relying upon the sun to heat water was common practice in the southwestern United States. Photos can be found showing pioneer families proudly showing off new homes equipped with solar water heaters. At one point, almost a quarter of the residents of Los Angeles relied upon the sun to heat their water with rooftop solar thermal systems.

The sun's heat can be used in two ways with homes and businesses. The sun is used to heat water for domestic hot water systems, or the sun's light can be concentrated and water temperatures increased to make steam and electricity.