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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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.