Resource assessment is the primary and essential exercise towards project evaluation. In renewable energy projects the resource assessment exercise is important because of the intermittent nature of the resource. The maximum possible value of solar radiation on earth is solar constant (i.e. 1367 W/m2 ); which is the part of the energy emitted by the sun per unit time intercepted by a unit area of surface perpendicular to the direction of propagation of the radiation at the earth's mean distance from sun outside the atmosphere. Hence this indicates the maximum level of solar radiation available close to the earth, while as experienced on the earth's surface, the intensity of incident solar radiation varies, due to earth's rotation and variation of distance from the sun throughout the year. The solar radiation intensity at any location varies throughout the year due to variation in solar declination angle, solar hour angle and geographical angles like latitude and longitude. Besides these atmospheric conditions, such as humidity, cloud cover, amount of dust in air etc contribute towards attenuation of solar radiation received on the earth's surface.