Watts, amps, and volts converter
Fill in two of watts, amps, and volts and the third is calculated. The two fields most calculators omit — power factor and phase — are the ones that make the difference on anything larger than a household circuit.
1.0 for resistive loads (heaters, incandescent). Motors typically 0.8–0.9.
The formula, and where the simple version breaks
For a single-phase resistive load, power equals volts times amps. That's the version every calculator implements, and it's correct for a space heater or an incandescent bulb.
It stops being correct in two common situations. On three-phase, power is √3 × V × I × PF — roughly 1.732 times the single-phase figure for the same voltage and current. And with any inductive load, the power factor means the current drawn is higher than the wattage alone suggests.
Get either wrong and you undersize the conductor.
Power factor in practice
Power factor is the ratio of real power doing work to apparent power drawn. Resistive loads are 1.0. Motors, transformers, and older fluorescent ballasts sit lower, commonly 0.8 to 0.9, because current and voltage fall out of phase.
The practical consequence: a 1,000 W motor at 0.85 power factor draws the current of about a 1,176 VA load. Utilities bill large customers on apparent power for exactly this reason.
Questions
Why does my answer differ from the nameplate?
Usually power factor or efficiency. A nameplate lists input current at rated conditions, including motor efficiency losses that a straight W = V × I calculation ignores. Where a nameplate figure exists, trust it over the calculation.
Do I use 120 V or 240 V?
Whatever the circuit actually is. In North American residential, general receptacles and lighting are 120 V; ranges, dryers, water heaters, and EV chargers are typically 240 V. A 240 V circuit delivers the same power at half the current, which is why large appliances use it.
What's the difference between watts and VA?
Watts is real power doing work; VA is apparent power the supply has to deliver. They're equal only at unity power factor. Conductors and breakers must be sized on current, which follows VA — not watts.