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Power calculator

Last reviewed 23 Aug 2026

Voltage

Current

Power 2,300 W

P = V × I = 230 × 10

Kilowatts 2.3 kW

Resistance 23 Ω

Current 10,000 mA

P = VI · AC adds power factor · 3-phase adds √3

Power is volts times amps for a DC or purely resistive load. On AC with a power factor, real power is V × I × pf for single phase and √3 × V × I × pf for three phase. A 400 V three-phase load at 16 A and 0.9 power factor draws 9.98 kW real from 11.09 kVA apparent.

How to calculate electrical power

1 Pick the pair of quantities you actually know.

2 For AC, set the power factor: 1 for heaters and lamps, 0.8–0.9 for motors.

3 Read real power in watts, and apparent power in VA if you are sizing a supply.

4 Use kVA rather than kW when sizing generators, transformers and UPS units.

The gap between watts and volt-amps is the single most misunderstood thing in electrical sizing. Watts is the power actually doing work; volt-amps is the product of the voltage and the current the supply has to deliver. For a resistive load they are equal. For a motor, current lags voltage and the two diverge by the power factor, so a 10 kW motor at 0.8 might pull 12.5 kVA. Cables, breakers and generators all have to be sized for the apparent power, because the current is real whether it is doing useful work or not.

Questions

What is the formula for electrical power? −

P = V × I for DC. For AC, P = V × I × power factor single phase, and P = √3 × V × I × power factor three phase.

What is the difference between kW and kVA? +

What power factor should I use? +

How do I convert watts to horsepower? +

Why is three phase √3 rather than 3? +

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