Image from J. Lean, Reviews of Geophysics, 29, 506, 1991.
Amount of radiant energy at all wavelengths received per unit time and area at the top of the earth's atmosphere, corrected to the mean earth-sun distance
customarily expressed in units of watts/m2
Total solar irradiance seems to fluctuate slightly
Best measurement to date is 1368 + 7 W/m2 obtained in 1980 using radiometers aboard NASA's Solar Max Mission (SMM) satellite.
