AC Power Factor Calculator
Calculate power factor, reactive power, and apparent power for AC circuits.
Determines if power factor correction is needed.
In AC circuits, the power factor (PF) measures how efficiently electrical power is being used:
PF = P/S = cos(φ)
Where:
- P = Real power (watts W) — the power actually doing work
- S = Apparent power (volt-amperes VA) — the total power supplied by the source
- φ = Phase angle between voltage and current
- Q = Reactive power (VAR) = √(S² − P²)
Power factor categories:
| Power Factor | Category | Notes |
|---|---|---|
| 1.0 | Unity | Pure resistive load (heaters, incandescent bulbs) |
| 0.95 – 1.0 | Excellent | Well-compensated industrial load |
| 0.80 – 0.95 | Good | Acceptable for most applications |
| < 0.80 | Poor | Utility may charge penalty fees |
| 0.5 or less | Very poor | Significant reactive power problem |
Why power factor matters:
A low power factor means the utility must supply more current than is actually needed for real work. This wastes energy in transmission line losses. That’s why:
- Industrial customers are billed for apparent power (kVA), not just real power (kW)
- Factories with electric motors use capacitor banks to correct low power factor
- Modern variable-frequency drives have near-unity power factor by design
Reactive power Q: Positive Q = inductive load (motors, transformers); current lags voltage Negative Q = capacitive load (capacitors); current leads voltage
Typical power factors by load type:
| Load Type | Power Factor |
|---|---|
| Resistive heater | 1.00 |
| Incandescent bulb | 1.00 |
| LED driver (good) | 0.90–0.99 |
| LED driver (cheap) | 0.50–0.70 |
| Electric motor (loaded) | 0.80–0.90 |
| Electric motor (unloaded) | 0.10–0.30 |
| Fluorescent lighting | 0.50–0.70 |
| Computer power supply | 0.60–0.99 |
| Air conditioner | 0.70–0.90 |
The dramatic difference between a loaded vs unloaded electric motor is why utilities and large facilities care so much about correction. A motor running near no-load is electrically wasteful: most of the apparent power it draws is reactive (magnetizing current) and does no useful work.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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