A common collector is called a flat-plate collector, and is usually mounted on the roof. This collector is a rectangular box with a transparent cover that faces the sun. Small tubes run through the box, carrying the water or other fluid such as antifreeze to be heated. The tubes are mounted on a metal absorber plate, which is painted black to absorb the sun's heat. The back and sides of the box are insulated to hold in the heat.
Heat builds up in the collector, and as the fluid passes through the tubes, it heats up.
The most common systems are active, which means they use pumps to move the heated fluid from the collector and into the storage tank. For that reason, homes also have a conventional backup system that uses fossil fuels. PV cells make electricity without moving, making noise, or polluting. They are used in calculators and watches. They also provide power to satellites, electric lights, and small electrical appliances such as radios.