What is power factor?

• A. The ratio of real power to apparent power.
• B. The ratio of apparent power to real power.
• C. The ratio of reactive power to apparent power.
• D. The sum of real power and reactive power.

Answer: (A)

Power factor tells you how effectively the power supplied by the source is doing useful work in the load. It is the ratio of real power to apparent power. Real power (P) is the actual work rate, measured in watts, that the load consumes. Apparent power (S) is the product of RMS voltage and RMS current, measured in volt-amperes, and it reflects the total power flow including reactive components. So PF = P/S, and it also equals cos(phi), where phi is the phase angle between voltage and current.

When the circuit is purely resistive, phi is zero and the power factor is 1, meaning all the power contributes to useful work. If reactive power is present due to inductive or capacitive elements, voltage and current are out of phase and the power factor drops below 1, indicating some power is circulating back and forth instead of doing work. The other statements describe different quantities: the reciprocal S/P, or Q/S (which relates to sin(phi)), or simply adding real and reactive power, none of which define power factor.