Power Factor Calculator
Power factor is a key measure of AC circuit efficiency. In a purely resistive circuit, voltage and current are always in phase and power factor equals 1.0. When reactive loads (inductors or capacitors) are present, voltage and current become out of phase, creating reactive power that does no useful work but still flows through the circuit. Power factor is defined as the cosine of the phase angle between voltage and current, or equivalently as the ratio of real power (watts, W) to apparent power (volt-amperes, VA). The power triangle shows that apparent power squared equals real power squared plus reactive power squared. This calculator lets you enter any two known quantities and derives all others. Select your input mode, enter values, and results update immediately. Use this calculator to diagnose power quality issues, size power factor correction capacitors, estimate utility demand charges, and verify transformer and generator sizing.
Power factor formulas
PF = Real Power (W) / Apparent Power (VA) = cos(θ)
VA = V × I
W = VA × PF
VAR = VA × sin(arccos(PF))
VA² = W² + VAR²
Worked example
230 V supply, 10 A load, power factor 0.85:
- Apparent power = 230 * 10 = 2,300 VA
- Real power = 2,300 * 0.85 = 1,955 W
- Reactive power = 2,300 * sin(arccos(0.85)) = 1,212 VAR
- Phase angle = arccos(0.85) = 31.79°
Frequently asked questions
What is power factor?
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amperes). It equals the cosine of the phase angle between voltage and current. A power factor of 1.0 means voltage and current are perfectly in phase; all apparent power is converted to useful work. A lower power factor means more current is drawn for the same useful power, increasing losses in wiring and distribution equipment.
What is the difference between real, reactive, and apparent power?
Real power (W) is the power that does useful work. Reactive power (VAR) is exchanged between the source and reactive loads (inductors, capacitors) and does no net work. Apparent power (VA) is the vector sum of real and reactive power. The relationship is: VA^2 = W^2 + VAR^2. Power factor = W / VA.
What causes a low power factor?
Inductive loads such as motors, transformers, and fluorescent light ballasts draw lagging reactive current, reducing power factor below 1.0. Capacitive loads produce leading reactive current. Most industrial sites have lagging power factor due to motor loads. Utilities often charge commercial customers a penalty for power factor below 0.9 or 0.95.
How do you correct a low power factor?
Power factor correction is achieved by adding capacitor banks in parallel with inductive loads. The capacitors supply reactive current locally, reducing the reactive current drawn from the utility. This lowers apparent power for the same real power, reducing utility bills and freeing up capacity in distribution equipment.
What is a typical good power factor for commercial equipment?
Most utility tariffs require a minimum power factor of 0.85 to 0.95. Well-designed equipment typically achieves 0.95 or higher. Modern switched-mode power supplies with power factor correction (PFC) circuits achieve 0.99 or better. Legacy equipment without PFC may have power factors as low as 0.5 to 0.7.
Sources
- IEEE Std 1459-2010: Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions.
- NIST: SI Units: Electric Current.
Reviewed by the CalculatorHub team, edited by James Graham. 14 June 2026.