Power Factor Calculator

Power factor is the ratio of real power to apparent power: PF = kW / kVA. It measures how much of the current your site draws is doing useful work. A power factor of 1.0 means every amp delivers real power; at 0.8, a fifth of the apparent power is reactive, circulating between the supply and your motors and transformers without doing work, but still loading cables, switchgear and your supply capacity.

For single-phase: PF = kW x 1000 / (V x I). For three-phase using line-to-line voltage: PF = kW x 1000 / (1.732 x V x I). The denominator is the apparent power in VA, so the division returns the ratio directly. For example, 50 kW measured on a 400 V supply drawing 85 A per line gives an apparent power of 58.9 kVA and a power factor of about 0.85.

Most UK businesses should aim for 0.95 or better. Distribution network operators apply reactive power charges to half-hourly metered sites whose power factor falls below the threshold for their region, commonly around 0.95, and a low power factor also wastes agreed supply capacity. Sites dominated by induction motors, welders or old fluorescent lighting often sit at 0.7 to 0.85 uncorrected, which is usually worth fixing with capacitor banks.

Reactive power (kVAR) is the power that magnetises motors and transformers. It does no useful work but the network still has to carry the current, so half-hourly bills meter it as kVArh and apply a charge when your power factor drops below your region's threshold. The exact threshold and charge depend on your distribution network operator and contract, so check the reactive power line on your bill rather than assuming.

Install power factor correction capacitors, either at individual large motors or as an automatically switched bank at the main board. The capacitors supply the magnetising current locally, so the network no longer has to. The size needed is the gap between your current kVAR and the kVAR at your target power factor: the reactive power calculator on this site works that out, including the capacitor size in microfarads.

They form a right-angled triangle: kVA squared = kW squared + kVAR squared. Real power (kW) does the work, reactive power (kVAR) magnetises inductive equipment, and apparent power (kVA) is what the supply actually has to deliver. The power factor is kW divided by kVA, and the phase angle between voltage and current is the angle of the triangle. Reduce the kVAR and the kVA shrinks towards the kW.