Which description correctly defines power factor?

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

Answer: (C)

Power factor describes how effectively electrical power is being converted into useful work. It is defined as the ratio of real power to apparent power: PF = P / S, where P is real power (watts) and S is apparent power (volt-amperes). Real power represents the portion that actually does work, while apparent power combines both the real work and the reactive power that circulates in the circuit. The power factor also equals the cosine of the phase angle between voltage and current, PF = cos(phi), ranging from 0 to 1. A PF close to 1 means most of the supplied power is doing useful work; a lower PF indicates more power is wasted in reactive effects.

So the description that defines power factor is the ratio of real power to apparent power. The other notions—reactive power to real power, the sum of real and apparent power, or apparent to real power—do not represent PF: Q/P is a different relationship, adding P and S isn’t a standard metric, and S/P is the reciprocal of PF.