Watts to amps calculator (and amps to watts)
Pick the direction, enter watts or amps, and choose the voltage. For AC devices you can add a power factor. If the device runs from a battery through an inverter, you'll also see the current the battery has to supply.
Running it from a battery through an inverter?
Quick reference
| Watts | at 120 V | at 240 V | at 12 V |
|---|
The formulas
AC: amps = watts ÷ (volts × power factor) · watts = amps × volts × power factor
Watts measure power, amps measure current, and volts measure the electrical "pressure" pushing that current. For the same power, a higher voltage needs fewer amps. That one idea explains why a 1,500 W heater draws a modest current from a wall outlet but a very large one from a 12 V battery.
What is power factor?
In AC circuits, some devices draw current that doesn't all turn into useful power. Power factor is the fraction that does, between 0 and 1. Heaters, kettles, toasters and incandescent bulbs are purely resistive, so their power factor is 1 and the simple formula works. Motors and many electronics have a power factor below 1, so they draw more amps than watts ÷ volts suggests. If a device's label lists both watts and amps, that difference is why. When in doubt, use the amps printed on the label.
How many amps is 1,500 watts?
| Supply | Calculation | Current |
|---|---|---|
| 120 V outlet | 1,500 ÷ 120 | 12.5 A |
| 240 V circuit | 1,500 ÷ 240 | 6.25 A |
| 12 V battery (DC) | 1,500 ÷ 12 | 125 A |
| 12 V battery through an 85%-efficient inverter | 1,500 ÷ (12 × 0.85) | about 147 A |
The last row is the one people underestimate. Running a 1,500 W appliance from a 12 V battery means roughly 150 A through the battery cables, which calls for very heavy cable and a large fuse. Our wire size calculator shows what that takes. It's also why bigger battery systems run at 24 V or 48 V, as explained in series vs. parallel.
Amps to watts: how much can a circuit handle?
Multiply the circuit's amps by its voltage to get the maximum watts: a 15 A, 120 V circuit can carry 15 × 120 = 1,800 W, and a 20 A circuit 2,400 W. Electrical codes generally size circuits so that loads running for three hours or more stay at or below 80% of the rating, which is 12 A, or about 1,440 W, on a 15 A circuit. A 1,500 W space heater running for hours sits right at that limit, so it's best on a circuit with nothing else heavy plugged in.
The same math works for portable power stations. An AC outlet rated "1,800 W continuous" can supply about 1,800 ÷ 120 = 15 A at 120 V. A 12 V car socket rated 10 A supplies at most 10 × 12 = 120 W.
Frequently asked questions
How many watts is 10 amps?
At 120 V, 10 A × 120 V = 1,200 W. At 12 V, 10 A × 12 V = 120 W. You always need the voltage to convert between amps and watts.
Can I convert watts to amps without knowing the voltage?
No. The same power can be any current depending on the voltage. In the US, standard outlets are 120 V. Vehicles and RV house batteries are usually 12 V. Check the label if you're unsure.
Why does my device's label show more amps than watts ÷ volts?
Usually because of power factor, or because the label lists the maximum current, for example at startup or at the lowest input voltage the device accepts. Use the label figure when sizing wires and fuses.
What about amp-hours?
Amp-hours (Ah) measure how much charge a battery stores, not how fast current flows. To compare batteries, convert amp-hours to watt-hours with the Ah to Wh converter.