A solar constant is a measurement of the solar electromagnetic radiation available in a meter squared at Earth's distance from the sun. The solar constant is used to quantify the rate at which energy is received upon a unit surface such as a solar panel. In this context, the solar constant provides a total measurement of the sun's radiant energy as it is absorbed at a given point.
Solar constants are used in various atmospheric and geological sciences. Though called a constant, the solar constant is merely relatively constant. The relative constant does vary by 0.2% in a cycle that peaks once every eleven years. The first attempt at estimating the solar constant was made by Claude Pouillet in 1838 at 1.228 kW/m2. The constant is rated at a solar minimum of 1.361 kW/m2 and a solar maximum of 1.362.
The entire spectrum of electromagnetic radiation is included in the measurement of a solar constant and not just that of visible light. The best direct measurements of the solar constant are taken from satellites. The Stefan-Boltzman constant can also be used as a means to calculate a solar constant. In this context, the constant defines the power per unit area emitted by a black body as a function of its thermodynamic temperature.