Watts to amps: the formula
For a DC circuit or an AC load that's purely resistive — heaters, toasters, kettles, incandescent bulbs — the conversion is one division:
Amps = watts ÷ volts
| Watts | Amps at 120 V | Amps at 240 V | Amps at 12 V DC |
|---|---|---|---|
| 100 W | 0.83 | 0.42 | 8.33 |
| 500 W | 4.17 | 2.08 | 41.7 |
| 1,000 W | 8.33 | 4.17 | 83.3 |
| 1,500 W | 12.5 | 6.25 | 125 |
| 1,800 W | 15 | 7.5 | 150 |
| 2,400 W | 20 | 10 | 200 |
| 3,000 W | 25 | 12.5 | 250 |
| 5,000 W | 41.7 | 20.8 | 417 |
The 12 V column is why inverters and RV wiring use heavy cables: a 1,000 W load on a 12 V battery pulls over 80 A.
Amps to watts
Watts = amps × volts. A 15 A circuit at 120 V delivers at most 1,800 W; a 20 A circuit, 2,400 W; a 30 A, 240 V dryer circuit, 7,200 W. Switch the calculator to Amps → watts to go this direction — useful when a nameplate lists only amps, or when you want to know how much one circuit can carry.
For motors, compressors and anything with a transformer, add the power factor: watts = volts × amps × power factor. A motor drawing 10 A at 120 V with a 0.85 power factor uses about 1,020 W, not 1,200. Three-phase equipment uses watts = 1.732 × volts × amps × power factor.
Watts to amps calculator for breaker sizing
The National Electrical Code treats a load that runs for three hours or more as continuous, and requires the circuit to carry 125% of it — which is the same as saying the load should be at most 80% of the breaker rating. That's where the familiar limits come from:
| Breaker | Max watts at 120 V | Continuous load (80%) |
|---|---|---|
| 15 A | 1,800 W | 1,440 W |
| 20 A | 2,400 W | 1,920 W |
| 30 A | 3,600 W | 2,880 W |
That's why a 1,500 W space heater belongs on its own 15 or 20 A circuit, and why two heaters on one circuit trip the breaker. The calculator shows the smallest standard breaker that keeps your load under 80%; your electrician also sizes the wire for its length and type.
To turn those watts into energy and cost, use the kWh calculator; to size a backup generator, the generator size calculator adds up running and starting watts.
