Light and heat from the Sun always pour down on the Earth. Turning this energy into electricity or using it as heat is called solar power. On a sunny day, a square patch of Earth facing the Sun with sides one metre long gets up to 1000 watts of power from the Sun, which is enough to run an electric heater. In fact, if we knew how to collect it and use it efficiently, the Sun could supply all the power we need for the whole world. The equipment needed to turn the Sun's energy into useful power is expensive, but once bought, the costs of running and maintaining that equipment are less than those of ordinary power-stations.

Solar radiation that is not absorbed or reflected by the atmosphere (for example, by clouds) reaches the surface of the Earth. The Earth absorbs most of the energy reaching its surface; a small fraction is reflected. In total, approximately 70% of incoming radiation is absorbed by the atmosphere and the Earth’s surface while around 30% is reflected back to space and does not heat the surface. The Earth radiates energy at wavelengths much longer than the Sun because it is colder. Part of this long-wave radiation is absorbed by greenhouse gases which then radiate energy into all directions, including downwards and thereby trapping heat in the atmosphere.