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That only works in cases where both the voltage and current waveforms are purely sinusoidal, which is rarely the case for the current. Measuring power factor accurately is complex, the cheap meters use a mathmatical approximation which is reasonably accurate as long as the peak-to-average value of the current isn't more than about 3X.You need to measure both the voltage waveform and current waveform (since you are making a wattmeter you would be doing this anyway) and compare the difference in phase.