1. Irradiance is the radiant flux (power) received by a surface per unit area. The SI unit of irradiance is the watt per square metre (W/m2).
2. The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.
radiant flux density
https://en.wikipedia.org/wiki/Irradiance (1.); http://en.openei.org/wiki/Definition:Irradiance (2.)
Author: Frithjof Kühnert