The available surface density of total solar radiation can be suitably calculated by the SOLEN method. The basic principles and relations of the mathematical model used in the SOLEN method are introduced. The universal SOLEN method enables calculation of the amount of solar energy incident on an arbitary tilted and orientated surface and transmitted through glazing for a chosen month. The SOLEN method uses the average monthly value of the relative sunshine duration, taking into account the mean real cloudiness of the respective geographical locality.