Solar systems principally use a so-called differential temperature controller. This control principle compares the temperatures at the absorber and in the storage tank. If the absorber temperature is a preset level above that of the storage tank, the circulation pump in the collector loop is switched on. The irradiation energy converted to heat in the absorber system is transported to the storage tank, whose temperature increases. When the temperature of the storage tank is equal to that of the absorber, no more energy can be supplied to the storage tank and the pump is switched off.