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. 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. Solar Design for Buildings One simple, obvious use of sunlight is to light our buildings.