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 It is the amount of energy received from the sun per second per unit area by a perfectly black body placed on the earth;the area being perpendicular to the direction of radiation.It is represented by the symbol S and its value is 1.388 x 10³ W/m² or 2 cal/cm²/minute.

S = 1.388 x 10³ W/m²

This means that at a distance of 1A.U. the sun's energy could raise the temperature of 1 cm² of water by 2 K in one minute.