The solar constant at Earth’s mean distance from the Sun is 1361 W/m².
Despite its name, the solar constant is not perfectly constant. It is the conventional mean total solar irradiance at one astronomical unit, measured above Earth’s atmosphere on a surface perpendicular to the incoming sunlight. Space-based instruments show that it changes slightly as the Sun’s activity rises and falls, and Earth’s elliptical orbit also changes the irradiance during the year.
The figure applies at the top of the atmosphere, not at the ground. Atmospheric gases, clouds and scattering reduce the sunlight reaching Earth’s surface. For the whole spherical Earth, the global average is approximately one quarter of the perpendicular value—about 340 W/m²—because only half the planet is illuminated at once and sunlight arrives at varying angles.
Older textbooks often quote 1,370 W/m², a historical rounded estimate. Values near 1,365 or 1,360 also appear in older sources, but 1361 W/m² is the modern standard mean value. It should not be confused with local daylight irradiance, which depends strongly on weather, latitude, season and solar angle.