This paper identifies weak points in the control of commercial DC variable-speed compressors, fed by photovoltaic panels, and describes the development of an energy-efficient control method. The method can be used in any application requiring such compressors to be driven by PV's, either for heating or cooling purposes. The advantage of using the presented method is the improved utilization of the PV electric energy, achieved by modifying the compressor startup characteristics and exploiting the maximum power of the PV.