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In an ac circuit, power factor is the cosine of the phase angle between the voltage and the current.

In a purely resistive circuit, the current and voltage are exactly in phase, so power factor is 1.00

In a purely capacitive or inductive circuit, the current and voltage are separated by 90 degrees (one is at a maximum while the other is zero).  In this case the power factor is 0 (cos 90 deg).

These are the two extremes. Real world power factor will be somewhere in between.

So if you know the magnitudes of current and voltage, as well as the phase angle between them:

Real Power = V*I*pf

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