Abstract: A method and apparatus for controlling the frequency of a power inverter for converting D.C. current into A.C. current having a frequency controlled by gating signals, which apparatus and method includes a load voltage detector for deriving a voltage wave; a load current detector for deriving a current wave; a phase shifter to shift the phase of one of the waves by 90.degree.; a multiplier to multiply the voltage and current waves; one with the other to produce a product; a reference signal for comparison with the product to create a control signal; and, a gating signal controller responding to the control signal.