insolation

Insolation is the solar radiation energy received by a surface per unit area over a specified period. It is the time-integrated value of solar irradiance, and is commonly expressed in units such as watt-hours per square meter (Wh/m²), kilowatt-hours per square meter (kWh/m²), or megajoules per square meter (MJ/m²). By contrast, irradiance is an instantaneous power flux, measured in watts per square meter (W/m²).

Insolation on a surface can be classified by its components. Direct insolation comes from direct sunlight arriving along a sun-ray path. Diffuse insolation comes from sunlight scattered by the atmosphere, clouds, and the surface. The total energy received on a surface is the sum of the direct component projected onto the surface and the diffuse component. On a horizontal surface, this total is often referred to in terms of global insolation if one speaks of energy over a time period.

Several factors influence insolation. The solar constant, about 1361 W/m² at the top of the atmosphere, is reduced before reaching the surface by atmospheric absorption and scattering. Insolation also depends on latitude, season, time of day, surface orientation, elevation, and atmospheric conditions such as clouds and aerosols. Because of these factors, annual mean insolation varies widely by location, and daily insolation can range from near zero at night or on cloudy days to several kilowatt-hours per square meter on bright, clear days.

Measurement and use. Insolation is estimated from pyranometer measurements or computed from solar geometry models and meteorological data. It is a key input for solar energy system design, crop and climate studies, and solar resource mapping.