Power Factor Calculator

Power factor is a key measure of how efficiently an alternating current circuit uses the electricity supplied to it, and this calculator works it out along with the related quantities from two simple figures. Enter the real power, the watts that actually do useful work, and the apparent power, the volt-amps the supply has to provide, and it returns the power factor, the phase angle between voltage and current, and the reactive power, updating as you type. The power factor is just the ratio of real to apparent power, a number between 0 and 1, and it equals the cosine of the phase angle, which is why electricians often call it cos phi. A power factor of 1 is ideal, meaning every volt-amp delivered is turned into useful work, while a low power factor means the supply is shuffling extra current back and forth that does nothing useful. That wasted current still has to be generated and carried, so it causes heating losses in cables and transformers, and for large commercial and industrial users it often attracts penalty charges from the lines company, which is why power factor correction, usually by adding capacitors, is such common practice. The reactive power, measured in volt-amps reactive, is the part that sloshes back and forth without doing work, and seeing it alongside the real and apparent power makes the power triangle, the right-angled relationship between the three, concrete. That makes the tool genuinely useful for electrical engineering and trades students learning about AC power and checking homework, and for electricians and facilities managers assessing loads, sizing correction equipment and understanding power bills. Real power in watts and apparent power in volt-amps give a power factor with no units. The formula and a worked example are explained clearly below.

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Apparent power should be at least the real power. Power factor near 1 is most efficient.