At the top of the earth’s atmosphere received insolation is expressed as the solar constant. It receives at the top of the atmospheric surface (thermopause) on a perpendicular plane to the solar beam. The average insolation received at the thermopause i.e.1368Wm2(Watt per squaremetre)energy(solar constant) is in the form of short wave. Thus, it is termed as solar constant for that mean distance from the sun. These solar constant is varying over 1 Wm2 by periodic disturbances and explosions in the solar surface basically related to sun spot. Sun spots are dark and cooler areas visible on the sun’s surface.The recent researches have shown that more and more energy is released when the sunpots are in large number.The number of sunspots also increases or decreases on a regular basis, creating a cycle of 11 years.