Insolation is a measure of solar radiation energy received on a given surface area and recorded during a given time. It is also called solar irradiation and expressed as hourly irradiation if recorded during an hour, daily irradiation if recorded during a day, for example. The unit recommended by the World Meteorological Organization is megajoules per square metre (MJ/m2) or joules per square millimetre (J/mm2) . Practitioners in the business of solar energy may use the unit watt-hours per square metre (Wh/m2). If this energy is divided by the recording time in hours, it is then a density of power called irradiance, expressed in watts per square metre (W/m2).
Read more about Insolation: Absorption and Reflection, Projection Effect, Earth's Insolation, Distribution of Insolation At The Top of The Atmosphere, Applications