The below mentioned article provides a formula for calculating power factor in the A.C. circuit.
The power in watt absorbed by a d.c. circuit is known when the supply pressure in volt across the circuit is multiplied by the current in ampere flowing through the circuit. But in an a.c. circuit the product of supply pressure and the circuit current gives the Apparent Power of the circuit. It is expressed in volt-ampere and not in watt. If the pressure applied across the circuit be V volt and the current flowing through the circuit be I ampere, the apparent power required for the circuit is VI volt-ampere.
The True Power or Actual Power absorbed by an a.c. circuit is VI cos θ watt, where θ is the phase angle between V and I.
Power factor of an a.c. circuit is the ratio = True Power / Apparent Power
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Thus, power factor = [(VI cosθ)/ VI] = cosθ
i.e. cosine of the phase single of an a.c. circuit is the power factor of that circuit. If the current lags behind the applied voltage, the power factor is called a lagging power factor; when the current leads the applied voltage, the power factor is called a leading power factor.
Sometimes power factor is expressed in per cent. In that case power factor indicates the true power expressed as a percentage of the apparent power. For example, if the power factor of an a.c. circuit is 80 per cent, it indicates that the true power absorbed by the circuit is 80 per cent of its apparent power, i.e.,
(True Power / Apparent Power) x 100 = 80%,
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or, True Power / Apparent Power = 0.8