Appliance Amp Draw Calculator (Watts to Amps)

Divide the power in watts by the voltage: amps = watts / volts. A 1,000 W device draws about 8.3 A on 120 V but 83.3 A on 12 V, because the lower the voltage, the more current is needed to move the same power. That is the single most important fact in DC system design: low-voltage appliances pull big currents, which is why 12 V wiring and fuses are so much heavier than their 120 V equivalents.

Power is voltage times current, so for a fixed wattage the current is inversely proportional to the voltage. Drop the voltage by a factor of ten and the current rises by a factor of ten. A 1,200 W microwave is about 10 A on a 120 V outlet but roughly 100 A drawn from a 12 V battery through an inverter. This is exactly why running large AC appliances off a 12 V battery requires very thick cable, large fuses and a substantial battery bank.

The amp draw is the starting input for both. Feed it into the DC Wire Size tool with your run length and voltage-drop target to choose the gauge, and pick a fuse or breaker rated just above the continuous draw (typically 1.25× for a continuous load) but below the wire’s ampacity. The fuse protects the wire, so it must always be sized to the conductor, not just the appliance. For motors, account for the higher starting surge as well.

For resistive loads — heaters, incandescent bulbs, kettles — the power factor is 1 and watts / volts is exact. Motors, compressors and some electronics have a power factor below 1, so the real current is watts / (volts × PF); divide by a PF of about 0.8 if you only have the real-power rating. When an AC appliance runs through an inverter from a battery, also add roughly 10% for inverter inefficiency when you work back to the battery current. This tool gives the ideal unity-PF figure.

Devices with motors — fridges, air conditioners, pumps — briefly draw two to three times their running current at startup. The amp draw here is the steady running current; for fuse, wire and inverter sizing on a motor load, plan for that surge using the Appliance Wattage tool or the appliance reference table. Continuous wiring is sized on running current, but the inverter and any soft-start budget must tolerate the much higher inrush for the first fraction of a second.

At 12 V, divide the watts by 12: a 100 W load draws about 8.3 A, a 300 W load about 25 A, and a 1,000 W load about 83 A. At 24 V those currents halve, and at 120 V they fall by a factor of ten. This inverse relationship is why higher-voltage systems are favored for big loads — the wire, fuses and connectors can be far smaller. Enter any wattage and voltage above to get the exact current for your own appliance and system.

The basic watts / volts relationship holds for both, but AC adds power factor. A purely resistive AC load (a heater) behaves like DC, so the simple division is exact. A reactive AC load (a motor or a switching power supply) draws more current than watts / volts suggests, because some current does no real work; for those, the true current is watts / (volts × power factor). This tool gives the ideal unity-power-factor value, which is exact for DC and resistive loads and a lower bound for reactive ones.