Power factor is defined as which of the following?

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

Answer: (B)

Power factor measures how effectively the supplied power is doing useful work. Real power P (watts) is the power that actually powers loads; apparent power S (volt-amperes) is the product of RMS voltage and current without regard to the phase angle; reactive power Q (vars) represents energy stored and released by inductors and capacitors.

The power factor is the ratio P divided by S, which equals cos(phi), where phi is the phase angle between voltage and current. It ranges from 0 to 1, with 1 meaning all the power is real and doing useful work. A lower PF means more current is needed to deliver the same real power and leads to greater losses and voltage drops; improving PF (for example with capacitors) reduces current for the same useful work.

So the correct definition is the ratio of real power to apparent power. The ratio of apparent power to real power would be the reciprocal of the power factor, and the sum of real and apparent power isn’t a standard power quantity.