One embodiment is a method of calibrating current offsets from a direct current bus current circuit adapted to control a motor including the steps of initializing the direct current bus current circuit, disengaging pulse width modulated signals to the motor, sampling current values at a predetermined sampling rate, averaging the current values over a predetermined number of samples to establish an offset value, and calculating a calibration value that will nullify the offset value in any subsequent processing of current samples. Another embodiment, a method of filtering inaccurate data from a sequential calculation of power values based on repeatedly sampled voltage and current including the steps of establishing a moving window of a predetermined number of power values, calculating an average power over a sequence comprising the predetermined number of power values, calculating an error value for each time, and comparing each error value to a preset value.