Sunlight is Earth’s primary source of energy. The only other source of energy the Earth has are the fissionable materials generated by the cataclysmic death of another star. These fissionable materials trapped in the Earth’s crust is what gives rise to geothermal energy, which drives the volcanism on Earth while also making it possible for mankind to fuel nuclear reactors. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1368 W/m2 (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth). Sunlight at the top of Earth’s atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.
Sunlight on the surface of Earth is attenuated by the Earth’s atmosphere so that less power arrives at the surface—closer to 1000 W/m2 in clear conditions when the Sun is near the zenith. The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths.
Solar energy can be harnessed by a variety of natural and synthetic processes—photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work, sometimes employing concentrating solar power (that it is measured in suns). The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.