Power Factor Calculator
Convert between real power (W), apparent power (VA), reactive power (VAR) and power factor (PF). Select two known values and the calculator derives the rest. Useful for UPS sizing, motor circuits, transformer loading and generator selection.
Last updated: May 2026
Select a mode and enter two values to calculate power factor.
PF = W / VA | VAR = sqrt(VA² − W²)
Understanding power factor in AC circuits
Power factor measures how efficiently an AC circuit converts apparent power (VA) into real work (W). A PF of 1.0 means all the power supplied does useful work. A PF of 0.8 means 80% does useful work; the remaining 20% circulates in the circuit as reactive power without being consumed.
The practical consequence: a device with a low power factor draws more current from the supply than its wattage alone suggests. A motor labelled 3 kW with a PF of 0.8 actually draws 3.75 kVA from the grid. You need to size the cable, fuse, and upstream circuit for 3.75 kVA, not 3 kW.
Typical power factor values by load type
| Load type | Typical PF | Reactive power source |
|---|---|---|
| Resistive heating, incandescent lamps | 1.00 | None (purely resistive) |
| LED driver with PFC | 0.90 to 0.99 | Minimal (corrected) |
| LED driver without PFC | 0.50 to 0.70 | Switching converter harmonics |
| Single-phase induction motor | 0.60 to 0.85 | Inductive (magnetising current) |
| Three-phase induction motor (full load) | 0.80 to 0.92 | Inductive |
| Transformer (no load) | 0.10 to 0.20 | Very high inductive reactive power |
| UPS (online double-conversion) | 0.80 to 0.95 | Depends on load and design |
| Computer switch-mode PSU (80 PLUS) | 0.95 to 0.99 | Minimal with active PFC |
Why UPS ratings are in VA, not W
A UPS is rated in VA because it must supply the full apparent power, including reactive power, regardless of the load's power factor. A 1000 VA UPS with an 800 W real-power rating assumes PF = 0.8. If you connect a purely resistive load of 900 W (PF = 1.0), the UPS is overloaded even though 900 W is below the 1000 VA label, because 900 W at PF 1.0 requires 900 VA. Always check both the VA rating and the W rating when sizing a UPS.
Frequently Asked Questions
My generator is rated in kVA. How do I convert to kW?
Multiply the kVA rating by the power factor to get kW. A 10 kVA generator with a typical generator power factor of 0.8 delivers 8 kW of real power. If your load has a better power factor (say 0.95), the same generator can support up to 9.5 kW of real load before reaching its VA limit. Generator specifications typically state the assumed power factor (often 0.8 lagging). If your loads are resistive heaters or have PFC-corrected power supplies, your generator can serve more wattage than the nameplate kW figure suggests.
What is reactive power and why does it matter for billing?
Reactive power (VAR) is the portion of apparent power that oscillates between the source and inductive or capacitive loads without being consumed. From the grid's perspective, reactive power still flows through cables and transformers, causing resistive losses. Industrial customers are often charged a reactive power penalty (kVAr surcharge) if their power factor falls below 0.9 or 0.95. Adding power factor correction capacitors in parallel with inductive loads cancels out the reactive current. Residential meters typically measure only real energy (kWh), so domestic customers are not directly billed for reactive power.
How do I improve a low power factor?
For inductive loads (motors, transformers, fluorescent ballasts), add capacitor banks in parallel. The capacitor's leading reactive power offsets the inductor's lagging reactive power, bringing the net power factor closer to 1.0. For variable loads, automatic power factor correction (APFC) panels switch capacitor stages in and out to track the load. For switched-mode power supplies and LED drivers, specifying equipment with active PFC (power factor correction) built into the front-end converter is the simplest solution at the device level. Capacitor correction is not effective against harmonic distortion from non-linear loads; harmonic filters or active front-end inverters are needed in those cases.
Can power factor be greater than 1?
No. Power factor is defined as W / VA, and real power (W) can never exceed apparent power (VA). A PF above 1.0 is physically impossible in a linear AC circuit. If a meter or calculation returns a PF above 1.0, there is a measurement error: the power transducer has incorrect wiring (current transformer polarity reversed), the instrument is not measuring true RMS for non-sinusoidal waveforms, or there is a data entry error. Check the inputs and wiring rather than accepting a result above 1.0.
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