The sun generates an enormous amount of energy — approximately 1.1 x 1020 kilowatt-hours every second. (A kilowatt-hour is the amount of energy needed to power a 100 watt light bulb for ten hours.) The earth’s outer atmosphere intercepts about one two-billionth of the energy generated by the sun, or about 1500 quadrillion (1.5 x 1018 ) kilowatt-hours per year. Because of reflection, scattering, and absorption by gases and aerosols in the atmosphere, only 47% of this, or approximately 700 quadrillion (7 x 1017 ) kilowatt-hours, reaches the surface of the earth.

Solar energy runs the engines of the earth. It heats its atmosphere and its lands, generates its winds, drives the water cycle, warms its oceans, grows its plants, feeds its animals, and even (over the long haul) produces its fossil fuels. It also runs the engines of our economies and of our society in general. We depend upon the energy from plants, water, wind, and fossil fuels to power our industries, heat and cool our homes and business, and run our transportation systems.