Solar power describes many methods of harnessing energy from the light of the Sun. It has been present in traditional building for centuries, but has become of increasing interest as the environmental costs and limited supply of other power sources such as fossil fuels are realized. As the Earth orbits the Sun, it receives approximately 1,400 W / m² of energy, as measured by the solar constant. Of the energy received, roughly 19% is absorbed by the atmosphere, while clouds on average reflect a further 35% of the total energy. The generally accepted standard is 1020 watts per square meter at sea level.