Power factor can be described as which ratio?

• 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 the current is being used to do useful work. It is the ratio of real (active) power to apparent power. Real power is the portion that actually powers loads, while apparent power is the total power flow, calculated from voltage and current, including the portion associated with reactive energy. In AC circuits, P = VI cos(phi) and S = VI, so PF = P / S = cos(phi). This value ranges from 0 to 1, with 1 meaning all the power is doing useful work (a purely resistive load). A lower PF means more current must flow to deliver the same amount of real power, increasing losses and size requirements for conductors and equipment. The ratio of apparent to real power would describe something different, and the sum of real and apparent power isn’t a ratio at all, so the correct description is the ratio of real power to apparent power.