Power Factor Explained

Power factor is the gap between the power your utility has to supply and the power your equipment actually uses. Resistive loads (heaters, incandescent bulbs) have a power factor of 1.0 and use everything supplied. Reactive loads (motors, transformers) and non-linear loads (SMPS chargers, LED drivers, VFDs) have a lower power factor, meaning the wire and breaker carry extra current that does no useful work. This page explains both the sinusoidal and non-linear cases and lists typical values by load type.

Formulas

PF = Real Power (W) ÷ Apparent Power (VA)

PF = cos(φ) for linear loads, where φ is the voltage-current phase angle (sinusoidal waveforms only)

True PF = displacement PF × distortion factor (for non-linear loads like SMPS, LED drivers, VFDs)

Watts = VA × PF

Power factor is the ratio of real power (watts, which does useful work) to apparent power (volt-amperes, which the supply must deliver). A PF of 1.0 means 100% of the apparent power is converted to real power; this is the case for resistive loads like heaters and incandescent bulbs. A PF of 0.85 means only 85% of the apparent power becomes real power, and the remaining 15% of VA must still be supplied through the wires and breaker even though it does no useful work. For sinusoidal linear loads (induction motors, transformers), the PF equals the cosine of the phase angle between voltage and current. For non-linear loads (switch-mode power supplies, LED drivers, variable-speed drives), the current waveform is distorted rather than just phase-shifted, so the true PF is the product of the displacement PF and a distortion factor, and the simple cos(φ) identity no longer applies. Either way, a lower PF means higher amps for the same real power, which pushes up wire and breaker size and can trigger utility penalty charges on commercial accounts.

Worked Examples

Example 1: Computer load with 1000 VA of apparent power at PF 0.65

Load's real power = 1000 × 0.65 = 650W (this is the load's real-power draw, not a UPS's output capacity)

Example 2: Motor drawing 10A at 240V (PF 0.80)

Apparent = 240 × 10 = 2400 VA. Real = 2400 × 0.80 = 1,920W

Example 3: Same motor if PF corrected to 0.95

Same 1,920W but now draws only 1920 / (240 × 0.95) = 8.42A instead of 10A

Common Mistakes

Using power factor for DC circuits. PF only applies to AC. DC circuits have PF = 1 by definition.
Assuming VA equals watts. They are only equal when PF = 1.0 (purely resistive loads).
Using typical PF values for precise engineering. Actual PF varies with load, speed, and conditions.

Try the Calculator

Use the interactive VA to Watts Calculator for instant results with any values. Every result page shows the formula applied with your specific numbers.

All Formulas

functionsWatts to Amps Formula functionsOhm's Law Formula functionsEnergy Cost Formula functionsWire Sizing Formula functionsVoltage Drop Formula functionsThree-Phase Power functionsHP to Amps Formula functionsSeries & Parallel functionsAC vs DC functionsElectrical Units functionsNEC Reference

This is a formula reference page for educational use. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before applying any of these formulas to real installations.

Standards & References

This page cites the following electrical codes and standards. Always consult the current edition of your local adopted standard for authoritative requirements.

NEC 210.19(A) Informational Note 4. Branch-circuit conductors sized to prevent a voltage drop exceeding 3% at the farthest outlet. Combined with feeders, total voltage drop should not exceed 5%.
National Electrical Code (NFPA 70), Article 210, Branch Circuits. Reference →
NEMA MG 1. National standard for motors and generators. Defines motor efficiency classes, power factor values, insulation temperature ratings, service factor, and locked-rotor currents. The industry reference for motor nameplate interpretation.
National Electrical Manufacturers Association. Reference →

Disclaimer: The information on this page is provided for reference. Always consult a licensed electrician and the current edition of your local adopted electrical code before performing electrical work.