Table of Contents
What Is Power Factor?
Power factor (PF) is the ratio of real power (watts) to apparent power (volt-amperes) in an AC circuit. It measures how effectively electrical power is being used. A power factor of 1.0 (unity) means all supplied power performs useful work. A power factor of 0.7 means only 70% of the current delivers real power, with the remaining 30% circulating as reactive current that creates magnetic or electric fields but does no useful work.
Low power factor wastes energy, requires larger conductors, increases utility bills (many commercial tariffs penalize low PF), and reduces transformer and generator capacity. Inductive loads like motors, transformers, and fluorescent ballasts cause lagging power factor, while capacitive loads cause leading power factor. Most industrial facilities target a power factor of 0.95 or higher through correction capacitors.
Power Triangle Formulas
Where S is apparent power (VA), P is real power (W), Q is reactive power (VAR), and phi is the phase angle between voltage and current.
Typical Power Factors
| Load Type | Power Factor |
|---|---|
| Resistive heaters | 1.0 |
| Incandescent lamps | 1.0 |
| LED drivers (with PFC) | 0.95-0.99 |
| Induction motors (full load) | 0.80-0.90 |
| Induction motors (no load) | 0.10-0.30 |
| Fluorescent lamps (magnetic ballast) | 0.50-0.60 |
| Welding machines | 0.50-0.70 |
Power Factor Correction
Capacitors are added in parallel to inductive loads to supply reactive power locally, reducing the reactive current drawn from the supply. The required capacitor size depends on the current and target power factor. Automatic power factor correction banks switch capacitor stages on and off to maintain the desired PF as loads change throughout the day.
Frequently Asked Questions
Why does low power factor increase electricity costs?
Utilities must generate and transmit apparent power (VA) even though they bill primarily for real power (W). Low PF means higher currents for the same real power, causing higher I-squared-R losses in transmission lines and requiring larger equipment. Many commercial tariffs include demand charges based on kVA or impose penalties for PF below 0.90.
Can power factor exceed 1.0?
No. Power factor ranges from 0 to 1. Over-correction with too many capacitors causes leading power factor, which can create voltage rise, resonance issues, and potential equipment damage. The target is typically 0.95-0.98, not exactly 1.0, to provide a safety margin against over-correction.
What is the difference between leading and lagging power factor?
Lagging PF means current lags behind voltage, caused by inductive loads (motors, transformers). Leading PF means current leads voltage, caused by capacitive loads or over-compensated circuits. Most industrial loads are inductive, making lagging PF the more common issue that requires correction.