A solar hot water panel is a solar water heater that uses the sun's energy to heat a fluid, usually water, which is used to transfer the heat to a heat storage vessel. In the home, for example, potable water would be heated and then stored in a hot water tank. Flat-plate solar-thermal collectors are usually placed on the roof, and have an absorber plate to which fluid circulation tubes are attached. The absorber, usually coated with a dark selective surface, assures the conversion of the sun's radiation into heat, while fluid circulating through the tubes carries the heat away where it can be used or stored. The heated fluid is pumped to a heat exchanger, which is a coil in the storage vessel or an external heat exchanger where it gives off its heat and is then circulated back to the panel