insolation

The amount of direct solar radiation striking per unit of surface at a given level. It usually refers either to the radiation reaching the top of the atmosphere or to that reaching the Earth's surface. It is expressed in W/m² and depends on the solar constant, the time of year, the latitude, and the degree of transparency of the air. The term can also refer to the intensity or total amount of solar radiation (direct + diffuse) incident per unit of horizontal surface, as well as to hours of sunshine; which in Meteorology is equivalent to the time during which, over the course of a day, the sun shines without being covered by clouds.