Themes > Science > Chemistry > General Chemistry > Energy > Solar Energy > Definition and Units 

Solar Energy, radiant energy produced in the sun as a result of nuclear fusion reactions  It is transmitted to the earth through space in quanta of energy called photons, which interact with the earth's atmosphere and surface. The strength of solar radiation at the outer edge of the earth's atmosphere when the earth is taken to be at its average distance from the sun is called the solar constant, the mean value of which is 1.37 × 106 ergs per sec per cm2, or about 2 calories per min per cm2.

The intensity is not constant, however; it appears to vary by about 0.2 percent in 30 years.

The intensity of energy actually available at the earth's surface is less than the solar constant because of absorption and scattering of radiant energy as photons interact with the atmosphere.

The strength of the solar energy available at any point on the earth depends, in a complicated but predictable way, on the day
of the year, the time of day, and the latitude of the collection point. Furthermore, the amount of solar energy that can be collected depends on the orientation of the collecting object