Amps to kW Calculator

How many kW in an amp?

One amp is worth a different number of kilowatts depending on the voltage, the phase and the power factor:

Multiply by your current to get a quick estimate. For example, 100A per phase on a 3 phase 400V supply at 0.8 PF is about 55.4 kW.

How to convert amps to kW

1. Find the current. Use a clamp-meter reading, the supply or breaker rating, or the equipment data plate.
2. Confirm the voltage and phase. In the UK that's usually 230V single phase or 400V 3 phase.
3. Choose a power factor. Use 0.8 for mixed loads with motors, or 1.0 for purely resistive loads like heaters.
4. Calculate. Single phase: kW = amps × volts × PF ÷ 1000. 3 phase: kW = 1.732 × amps × volts × PF ÷ 1000.

Sizing a generator from measured amps

If you can measure your load, converting amps to kW is one of the most reliable ways to size a generator:

1. Measure each phase at peak times with a clamp meter, ideally while your largest equipment is running.
2. Take the highest phase reading. The generator has to carry the most heavily loaded phase.
3. Convert to kW with the calculator above.
4. Add headroom. For example, a highest reading of 120A per phase at 400V and 0.8 PF is 66.5 kW. Adding 25% gives 83.1 kW, so a 100 kW generator is the next size up.

To see the result in kVA rather than kW, which is how many generators are rated, use our Amps to kVA calculator.

Amps to kW vs amps to kVA

Converting amps to kVA gives the apparent power and needs no power factor. Converting amps to kW gives the real power, the useful work being done, so it needs the power factor. For generator sizing, check both: the kW against the engine rating and the kVA against the alternator rating. ATS panels and changeover switches are rated in amps, so keep your original amp figure to hand when choosing switchgear.