Available solar radiation at a given location – latitude, longitude and height above sea level – needs to be determined as a basis for any calculations of how much energy may usefully be harvested for a project. It is usually modelled by totalling three components: direct beam shortwave radiation, sky diffuse and ground reflected radiation. 1 The available extraterrestrial radiation can be calculated using astronomical formulae and the empirical solar constant (1,367 Wm−2).