1. A method of determining frequency deviation in the sinusoid wavefrom output of electrical power generation apparatus characterized by the fact that the method is accurate in the presence of noise and that it minimizes error introudced by such noise comprisng the steps of: successively sampling the voltage output of the sinusoid waveform of an electrical power generation apparatus; recursively electronically calculating in response to said successive sampling a voltage waveform sinusoid estimate by optimal estimation using state variables, said optimal estimation including electronically calculating the magnitude and phase angle of said sample from said state variables; and electronically calculating the frequency deviation of said voltage waveform by linearly relating the frequency deviation to the average rate of change of said phase angle.