Power Factor Correction Calculator
The reactive power (kVAr) of capacitors needed to lift a site’s power factor from where it is to a target, with the apparent power and current before and after.
Motors and transformers draw reactive current that does no work but loads the cables and the supply; the ratio of real to apparent power is the power factor.
How the power factor correction calculator works
Motors and transformers draw reactive current that does no work but loads the cables and the supply; the ratio of real to apparent power is the power factor. Utilities charge for low power factor or for kVA demand, so sites add capacitors to supply the reactive power locally. The capacitor size is P × (tan φ₁ − tan φ₂), where φ is the angle whose cosine is the power factor.
Formula: kVAr = P × (tan(arccos PF₁) − tan(arccos PF₂)); S = P ÷ PF
Worked examples
| Inputs | Capacitor bank needed (kVAr) | Note |
|---|---|---|
| 100 kW from 0.75 to 0.95 | 55.3 | 55.3 kVAr |
| 500 kW from 0.8 to 0.98 | 273.5 | 273.5 kVAr |
| 20 kW single-phase workshop from 0.7 to 0.92 | 11.9 | 11.9 kVAr |
FAQFrequently asked questions
Why not correct to 1.0?
The last few points cost the most capacitance for the least gain, and over-correction (a leading power factor) brings its own problems — voltage rise and resonance with harmonics. 0.95–0.98 is the usual target, often set by the utility’s tariff.
How is the bank installed?
As switched steps controlled by a power-factor relay, so the correction follows the load; a fixed bank sized for the average load risks over-correcting at night. Sites with variable-speed drives need detuned (reactor-protected) banks.
What does it save?
Whatever the tariff charges for kVA demand or for power factor below a threshold, plus lower losses in the site’s own cables and transformer. Payback is typically one to three years on demand tariffs.
Does it reduce my kWh?
Only slightly, through lower cable losses. Power factor correction reduces current and apparent power, not the real energy the equipment uses.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- Ofgem — the UK energy regulator
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.