Solar radiation is a term used to describe visible and near-visible (ultraviolet and near-infrared) radiation emitted from the sun. The different regions are described by their wavelength range within the broad band range of 200 to 100,000 nm (nanometers).

Terrestrial radiation is a term used to describe infrared radiation emitted from the earth. The components of solar and terrestrial radiation and their wavelength ranges are:
 

Approximately 99% of solar, or short-wave, radiation at the earth's surface is contained in the region from 300 to 3,000 nm while most of terrestrial, or long-wave, radiation is contained in the region from 3500 to 50,000 nm.

Outside the earth's atmosphere, solar radiation has an intensity of approximately 1370 watts per square meter. This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant. On the surface of the earth on a clear day, at noon, the direct beam radiation will be approximately 1000 watts per square meter for many locations.

The availability of energy is affected by location (including latitude and elevation), season, and time of day. All of which can be readily determined. However, the biggest factors affecting the available energy are cloud cover and other meteorological conditions which vary with location and time.

Historically, solar measurements have been taken with horizontal instruments over the complete day. In the Northern US, this results in early summer values 4-6 times greater than early winter values. In the South, differences would be 2-3 times greater. This is due, in part, to the weather and, to a larger degree, the sun angle and the length of daylight.