Over 70% of the energy consumption of the typical house in the UK is used for heating. An alternative to using increasingly expensive mains electricity is to use the Sun's own radiant energy. This is especially common in southern Europe where the intensity of the Sun's radiation is much greater than in the UK, so the payback time is quicker. A typical arrangement is shown below.
Short wavelength radiation from the Sun passes through a flat glass panel to be absorbed by blackened copper pipes. It passes through the pipes by conduction to heat the water inside. The longer wavelength radiation emitted by the water (longer wavelength because the water is cooler) does not pass through the glass panel. Much of it is reabsorbed by the water.
The heating effect of the Sun can be supplemented by mains electric heating – the whole thing being controlled by pumps and electronics, so that hot water is available when required. The heated water can also be used in the house's central heating system.