The amount of solar radiation received outside the earth's atmosphere on a surface normal to the incident radiation, and at the earth's mean distance from the sun. Reliable measurements of solar radiation can only be made from space and the precise record extends back only to 1978. The generally accepted value is 1368 W m⁻² with an accuracy of about 0. 2%. Variations of a few tenths of a percent are common, usually associated with the passage of sunspots across the solar disk. The solar cycle variation of TSI is on the order of 0. 1%.