## Solar Constant

The magnitude of solar irradiance at one Astronomical Unit (AU) is known as the solar constant. The approximate average value is 1.3608 kW/m², which is 81.65 kJ/m² per minute, is equivalent to approximately 1.951 calories per minute per square centimeter, or 1.951 langleys per minute. A calorie is the amount of energy, if absorbed completely, needed to raise the temperature of 1 cubic centimeter of water 1 degree Centigrade. At most about 75% of the solar energy actually reaches the earth's surface at the subsolar point at any one time during daylight. When taking into account the variation of day length across a hemisphere and angle of solar incidence, the solar energy arriving on a surface with unit area at the top of the atmosphere will be around 350 W/m² .

Solar output is nearly, but not quite, constant. Total solar output is now measured as varying (over the last three 11-year sunspot cycles) by approximately 0.1%.