A solar water heating (SWH) system - sometimes called solar thermal - consists of three major components: one or more solar collectors (tubes or panels), usually mounted on a roof, a pump connected to a controller, and a hot water cylinder with two heating coils - known as a twin-coil cylinder.
The collector uses the solar energy from sunlight to heat a thermal fluid (water with antifreeze). If the fluid in the collector is hotter than the water in the cylinder then the controller activates the pump to pump the fluid through the coil in the cylinder, thus heating water in the cylinder for showers and hot water taps.
The second heating coil in the cylinder is connected to a back-up boiler (e.g. gas boiler) to provide hot water when there isn’t enough sunshine.