solar constant

Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber.

The solar constant (denoted as S☉ by the IAU standard) is a mathematical equation averaging the amount of energy received by a given area one astronomical unit away from the Sun. To put simply, it is measuring the amount of energy per second per square meter that arrives from the Sun at the distance of Earth's orbit. More specifically, it is a flux density measuring mean solar electromagnetic radiation (total solar irradiance) per unit area. It is measured on a surface perpendicular to the rays, one astronomical unit (au) from the Sun (roughly the distance from the Sun to the Earth).