 Some of that energy has been preserved as fossil energy, some is directly or indirectly usable; for example, via solar PV/thermal, wind, hydro- or wave power.The total solar irradiance is measured by satellite to be roughly 1361 watts per square meter "(see solar constant)", though it fluctuates by about 6.9% during the year due to the Earth's varying distance from the sun.This value, after multiplication by the cross-sectional area intercepted by the Earth, is the total rate of solar energy received by the planet; about half, 89,000 TW, reaches the Earth's surface.