Amps to Watts Calculator

How to use the amps to watts calculator

Enter a current in amps and the voltage, and you get the power in watts — the maximum, plus the safe continuous wattage. Where the watts-to-amps tool answers “will this device fit on my circuit,” this one answers the opposite and equally practical question: given a circuit or outlet’s amp rating, how many watts can it actually power? There’s no rate here — this is a power relationship; the cost tools handle dollars.

The math is watts = amps × volts, exact for DC and resistive loads. (A power factor below 1 means a motor delivers slightly fewer real watts than this for the same amps — see the note.) A standard 15 A, 120 V circuit can carry 1,800 watts at its full rating; a 20 A circuit, 2,400 watts. That’s the ceiling before the 80% safety margin comes off.

Budgeting a circuit is the real use. You know the breaker — 15 or 20 amps — and you want to know what you can run on it at once. Take the watts here, apply the 80% continuous rule, and you have a usable power budget. A 15 A circuit’s safe continuous budget is 1,440 watts: enough for a 1,500 W heater? Not quite — which is why heaters and 15 A circuits are a classic nuisance-trip pairing.

The 80% rule shows up here too. Circuits are sized so continuous loads stay at 80% of the rating, so the tool gives both the maximum (amps × volts) and the safe continuous figure (80% of it). Plan to the continuous number for anything that runs more than a few hours — space heaters, ACs, EV charging — and keep the full maximum in mind only for brief loads.

120 versus 240 volts doubles everything. The same amps deliver twice the watts at 240 V: a 30 A, 240 V dryer circuit carries 7,200 watts, while a 30 A, 120 V circuit carries only 3,600. Match the voltage to the actual circuit. Knowing the watts a circuit can power is how you decide whether a new appliance can share an existing line or needs its own.

The formula

Power is current times voltage. Exact for DC and resistive loads; motors deliver slightly fewer real watts for the same amps, as the note explains:

watts = amps × volts

Worked example with the defaults — 10 A on a 120 V circuit: 10 × 120 = 1,200 watts maximum. Apply the 80% continuous rule and the safe sustained load is 960 watts. On a 240 V circuit the same 10 A delivers 2,400 watts.

A practical circuit budget: a 15 A, 120 V circuit maxes at 1,800 W but should carry only 1,440 W continuously; a 20 A circuit, 2,400 W max and 1,920 W continuous. That 80% margin is why a 1,500 W heater can nuisance-trip a 15 A circuit that’s also feeding a few other things.

Frequently asked questions

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