Watts to amps calculator
Convert watts to amps or amps to watts for 120 V, 230 V, 240 V or three-phase supplies, and work out energy use in kWh. Formulas and worked examples.
- Energy used
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Single phase: P = V × I × PF. For 400 V three-phase, P = √3 × V × I × PF (handled automatically).
- Single phase: amps = watts ÷ volts for resistive loads such as heaters and kettles.
- A 1,500 W space heater on a 120 V US circuit draws 12.5 A; a 3,000 W UK kettle on 230 V draws about 13 A.
- Watts measure instantaneous power; kilowatt-hours measure energy over time, which is what you pay for.
The formulas
- Watts = volts × amps × power factor
- Amps = watts ÷ (volts × power factor)
- kWh = watts × hours ÷ 1,000
The power factor is 1 for resistive loads that turn electricity into heat: heaters, kettles, toasters, ovens. Motors and some electronic power supplies have a power factor below 1; it is sometimes printed on the rating plate.
For a three-phase 400 V supply, power = √3 × 400 × amps × power factor. The calculator applies the √3 factor automatically when you choose 400 V.
Worked examples
| Appliance | Voltage | Current |
|---|---|---|
| Space heater 1,500 W (US) | 120 V | 12.5 A |
| Kettle 3,000 W (UK) | 230 V | 13.0 A |
| Electric radiator 2,000 W (Europe) | 230 V | 8.7 A |
Which voltage should I use?
- United States and Canada: 120 V for standard outlets; large appliances (dryers, ranges) often use 240 V circuits.
- United Kingdom: the nominal supply is 230 V (often still described as 240 V). The difference changes the result by only about 4%.
- Continental Europe: 230 V single phase, 400 V for three-phase supplies.
Watts versus kilowatt-hours
Watts tell you how fast an appliance uses energy. What you pay for is energy: power multiplied by time, in kWh. A 2,000 W heater running for 30 minutes uses 1 kWh, the same as a 10 W LED bulb left on for 100 hours. Our explainer watts vs kilowatt-hours goes further, and the appliance running cost calculator turns kWh into money.
Safety
This converter gives estimates. Choosing a cable size, breaker or socket is governed by wiring regulations (the NEC in the US, BS 7671 in the UK): if in doubt, ask a qualified electrician. Never exceed the rating of an extension lead or adapter.