Power Factor Calculator
Calculate power factor, reactive power (VAR), apparent power (VA), and phase angle from real power and any one additional known value.
Calculate Power Factor
Enter real power (W) and apparent power (VA) to find power factor and reactive power.
How to Calculate Power Factor
Power factor (PF) is calculated by dividing real power (watts) by apparent power (volt-amperes): PF = P ÷ S = cos(θ). Power factor ranges from 0 to 1, where 1.0 means all supplied power performs useful work (purely resistive load) and 0 means no real power is delivered (purely reactive load).
Scenario: A machine draws 8,000 W real power and 10,000 VA apparent power
PF = 8,000 ÷ 10,000
PF = 0.80 (80%)
The power factor is 0.80, meaning 80% of the supplied power does useful work.
The Power Triangle Explained
The power triangle is a right triangle that shows the relationship between real, reactive, and apparent power:
- Horizontal side (P) = Real Power in Watts — performs useful work
- Vertical side (Q) = Reactive Power in VAR — energy stored and released by inductors/capacitors
- Hypotenuse (S) = Apparent Power in VA — total power the source must supply
- Angle (θ) = Phase angle between voltage and current waveforms
The relationship: S² = P² + Q². A higher power factor means less reactive power and more efficient use of the electrical supply. Utilities often charge penalties for PF below 0.85–0.90.
Power Factor Reference Values
Typical power factor values by load type:
| Load Type | Typical PF | Phase Angle | Classification |
|---|---|---|---|
| Resistive heaters, incandescent bulbs | 1.00 | 0° | Unity |
| LED lighting with PFC | 0.95–0.99 | 2–18° | Excellent |
| Modern server PSU (80+) | 0.90–0.99 | 2–26° | Good |
| Industrial motors (loaded) | 0.80–0.90 | 26–37° | Average |
| Industrial motors (no load) | 0.15–0.35 | 70–81° | Poor |
| Fluorescent lamps (no PFC) | 0.50–0.65 | 49–60° | Poor |
| Welding machines | 0.40–0.60 | 53–66° | Poor |
Frequently Asked Questions About Power Factor
What is a good power factor?
A good power factor is 0.90 or higher. Most utilities consider PF above 0.95 excellent. PF between 0.85–0.95 is acceptable. Below 0.85 is poor and may incur penalty charges from the utility company. Unity power factor (1.0) is ideal but only occurs with purely resistive loads.
Why does low power factor increase electricity costs?
Low power factor means more current is drawn for the same real power, increasing I²R losses in cables, transformers, and generators. Utilities must supply this extra current, so they charge reactive power penalties or demand charges for commercial customers with PF below 0.85–0.90.
How can I improve power factor?
Power factor is improved by adding power factor correction (PFC) capacitors to offset inductive reactive power. Capacitors supply reactive current locally, reducing the total current drawn from the utility. Modern variable frequency drives (VFDs) and active PFC circuits in power supplies also improve PF close to unity.
What is reactive power (VAR)?
Reactive power (Q) is the power that oscillates between the source and reactive components (inductors and capacitors) without doing useful work. Measured in VAR (volt-amperes reactive), it represents energy stored in magnetic fields (inductors) or electric fields (capacitors) and returned to the source each cycle.
What causes poor power factor?
Inductive loads are the main cause of poor power factor. Electric motors, transformers, welding machines, and fluorescent ballasts all create lagging reactive power (inductive VAR) that reduces PF. Lightly loaded motors have the worst PF (as low as 0.15) because magnetizing current remains constant while real power output drops.
What is the phase angle?
Phase angle (θ) is the angular difference between the voltage and current waveforms in an AC circuit. θ = arccos(PF). At PF 1.0, voltage and current are in phase (θ = 0°). At PF 0.5, they are 60° apart. A larger phase angle means more reactive power and lower efficiency.