A blackened roof measures 20 m by 50 m. If the temperature of the sun's surface is 6000K, Stefan's constant (σ) is 5.7 x 10-8 Wm-2K-4, radius of the sun is 7.5 x 108 meter and the distance of the centre of the sun from the earth is 1.5 x 1011meter. Determine how much solar energy is incident on the roof per minute, suppose that half is lost in passing through the earth's atmosphere and the roof is normal to the sun rays.