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.
Why motors and UPS sizing need power factor
Power factor sits adjacent to the main circuit chain, but it changes the numbers at every upstream step. Before a new circuit comes together, you size the conductor for the current the load actually draws, then confirm voltage drop on the run and protect the cable with a fuse or breaker rated to its ampacity. You are here: correct watts to VA for reactive loads. That correction matters because a generator or UPS is rated in VA, not watts: a motor running at 0.7 power factor needs a supply far larger than its wattage suggests, so if you skip this step you can wire and fuse a circuit correctly by the watt figure and still overload the generator the moment you start the motor. Once you have the VA figure, the power supply calculator lets you size the supply against it, and the appliance running cost and electricity cost calculators turn the corrected load into a monthly figure.
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.
My meter showed a power factor reading above 1.0 on a motor circuit. What went wrong?
Power factor cannot exceed 1.0: it is defined as W / VA, and real power can never exceed apparent power. A reading above 1.0 on a motor circuit almost always points to a wiring or instrument fault, not a real condition. The most common causes are a current transformer (CT) with reversed polarity, a wattmeter with incorrect phase wiring between the voltage and current inputs, or an instrument that is not measuring true RMS on a non-sinusoidal waveform. Check the CT orientation and the phase connections first, then verify the instrument is rated for the harmonic content of the load before trusting any result above 1.0.
Methodology and sources
This tool relates real power (W), apparent power (VA), reactive power (VAR) and power factor (PF) using the AC power triangle. Give it any two known quantities and it derives the rest.
- Method: Power factor PF = W / VA. Apparent power VA = W / PF, and real power W = VA × PF. Reactive power VAR = sqrt(VA² − W²), which is the right-angle relationship S² = P² + Q² of the power triangle.
- Standards and sources: Standard AC power theory (the real / reactive / apparent power triangle). This is established electrical physics rather than a single named standard; the displacement-PF convention follows the IEC definition of power factor as the ratio of real power to apparent power.
- Assumptions and limits: Assumes a single sinusoidal supply and treats power factor as a number between 0.01 and 1.00, where W cannot exceed VA. It models displacement power factor only and does not account for harmonic distortion from non-linear loads, so for distorted (non-sinusoidal) currents the true PF will differ from this geometric estimate.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning and load-analysis aid, not a substitute for a qualified electrician or your local wiring and building code; size cables, fuses and equipment to the applicable code, not to this estimate alone.
Next step in this workflow
Know your power factor: now calculate true energy cost.