The method for calculation of the energy generated by solar radiation that can be used for heating in passive and active heat recovery systems is proposed. The calculation method refers to a specified position of a sun-lit object (windows or greenhouses, sun collectors), its shading by neighbouring objects, and also pertains to the specifics of the calculation method of solar energy that can be used for heating of buildings and greenhouses. The calculation algorithm includes the location of a sunlit object, its shading by neighbouring objects and also specifics of glazing.