An electric generator driven by a source of variable energy produces at its output an alternating current (AC) output voltage and current which is converted via controllable active switching circuitry into a direct current (DC) voltage which is stored in an energy storage element. Power losses are associated with the active switching circuit and the AC to DC conversion. The power available at the output of the generator can be determined. If the available power is less than the power losses the controllable switching circuitry is disabled.