De berekening is op basis van de straling:
Sunshine for any point is assessed using the model representation of cloud layers to decide how much direct solar (shortwave) radiation reaches the Earth's surface. The accumulated sunshine duration at each point, for a 24 hr period immediately prior to the validity time, is calculated from the length of time for which the direct solar (shortwave) radiation at the Earth's surface, falling on a plane perpendicular to the direction of the Sun, is greater than or equal to 120 W/m2. The minimum solar intensity level of 120 W/m2 is defined by the World Meteorological Organisation and is consistent with observed values of sunshine duration from a Campbell-Stokes recorder (sometimes called a Stokes sphere) which can only measure moderately intense sunlight and brighter.
Sunshine values are reasonable if there is continuous full cover of cloud (zero sunshine) or continuous cloud-free skies (sunshine totals approaching the maximum for the latitude and time of year). Partial cloud cover reduces direct sunshine accordingly. It is suspected that the amount of sunshine is over-estimated because the amount of cloud cover is only an average and cloud cover is not necessarily constant or uniformly distributed in time or space.