The first and most basic forms of solar water heaters were simply metal containers, such as barrels or drums, painted black. As solar radiation (also known as sunshine) was absorbed by these containers, it heated the water inside of them.

The downside to these early models was that they generally took all day to heat the water to a usable temperature, and once the sun went down the tanks quickly cooled.

Modern solar water heaters use the same principle but have incorporated a sophisticated system of pumps, storage containers, temperature gauges, anti-freeze valves, and collection devices to maximize the amount of the sun’s energy absorbed, hot water generated, and energy retained.

They typically cycle hot water into an existing conventional tank-style hot water heater, delivering preheated water, and thus reducing the energy load needed to maintain the hot water.