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 is the portion of electrical power that does useful work, expressed as the ratio of real power to apparent power. Real power (P) is the actual work done by the circuit and is measured in watts. Apparent power (S) is the product of voltage and current, measured in volt-amperes, and represents the total power that the system must handle. In AC circuits, the relationship P = VI cos(phi) holds, where phi is the phase angle between voltage and current. Since S = VI, the power factor is P/S = cos(phi). This means the power factor tells you how much of the apparent power is being converted into real work. A power factor of 1 (cos(phi) = 1) means voltage and current are in phase and all the power is real; a smaller power factor indicates some reactive power is circulating (due to inductors or capacitors), which requires more current for the same amount of real power and can cause inefficiencies. Reactive power is represented by Q = VI sin(phi). In practice, improving power factor (toward 1) reduces losses and improves efficiency.