Solar energy, power from the sun, is free and inexhaustible. In the broadest sense, solar energy supports all life on earth and is the basis for almost every form of energy we use. The sun makes plants grow, which are burned as fuel or rot in swamps and are compressed underground for millions of years to become coal and oil. Heat from the sun causes temperature differences between areas, causing the wind to blow. Water evaporates because of the sun, falls on high elevations, and rushes down to the sea, spinning turbines as it passes. But solar energy usually refers to ways this energy can be used as heat, lighting, and electricity.

The Solar Resource

The amount of energy from the sun that falls on the earth is enormous. All the energy stored in the earth’s reserves of coal, oil, and natural gas is matched by the energy from 20 days of sunshine. Outside the earth’s atmosphere, the sun’s energy contains about 1,300 watts per square meter. About one-third of this light is reflected back into space, and some is absorbed by the atmosphere (in part causing winds to blow).

By the time it reaches the earth’s surface, the energy in sunlight has fallen to about 1,000 watts per square meter, at noon, on a cloudless day. Averaged over the entire surface of the earth, 24 hours per day for a year, each square meter collects energy approximately equivalent to a barrel of oil. So each day, on average, a square meter collects 4.2 kilowatt-hours of energy.

This figure varies by location and by weather patterns. Deserts, with very dry air and little cloud cover, receive the most sun: more than 6.0 kilowatt-hours per day per square meter. Northern climes, such as that of Boston, get closer to 3.6 kilowatt-hours. Sunlight varies by season as well, with some areas receiving very little sunshine in the winter. Seattle in December, for example, gets only about 0.7 kilowatt-hours per day.

This solar resource is the maximum available to be captured and used for energy. A solar collector captures only a portion of this, depending on its efficiency. For example, a one square meter solar electric panel that has an efficiency of 15 percent would produce about one kilowatt-hour of electricity per day in Arizona.

Article provided by: Union of Concerned Scientists - www.ucsusa.org