Insolation's a measure of solar radiation energy received on a given surface area in a given time. It's commonly expressed as average irradiance in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kW·h/(m2·day)) (or hours/day). In the case of photovoltaics it's commonly measured as kWh/(kWp·y) (kilowatt hours per year per kilowatt peak rating). The given surface may be a planet, or a terrestrial object inside the atmosphere of a planet, or any object expo… (More on Insolation)