In a solar system collector loop, having a solar collector with an absorber plate, a heat storage unit and a pump for circulating a fluid between the collector and the storage unit, the stagnation temperature T.sub.Q of the collector absorber plate, the temperature T.sub.C of the collector absorber plate, and the storage temperature T.sub.S in the storage unit are measured. The stagnation temperature is measured by a heat flux transducer based on a small sample of the absorbed plate which is thermally isolated from the collector but in the same enclosure. This transducer generates temperatures near the stagnation condition for the instantaneous operating point. In the method of control, the pump is turned on when the temperature difference between the absorber plate T.sub.C or T.sub.Q and the storage T.sub.S, is greater than a predetermined value T.sub.on. The pump is turned off when the temperature difference between the stagnation temperature T.sub.Q and collector absorber plate T.sub.C is smaller than a predetermined value T.sub.off. The temperature signals T.sub.Q, T.sub.C and T.sub.S may also be used to control obvious abnormalities in the system as well as fast radiation changes such as those caused by small clouds.