The solar constant is solar irradiance on a plane normal to incoming sunlight at a specified reference distance from the Sun, conventionally the mean Earth-Sun distance. Local horizontal irradiance includes solar geometry and atmospheric transmission.
The solar constant is the measure of the amount of solar electromagnetic radiation (solar energy) received per unit area at a distance of one astronomical unit (AU) from the Sun, in a plane perpendicular to the rays of the Sun. As of current understanding, the solar constant is approximately 1,361 watts per square meter (W/m²).
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