Watts to Amps Calculator

To convert watts to amps, divide watts by volts: a 1,200W microwave on a 120V outlet draws 10 amps. The same 1,200W load running off a 12V battery pulls 100 amps, because current rises as voltage falls. Picking the right voltage is the whole job.

The formula, and what each term means

The relationship is amps = watts ÷ volts, and it is exact for any DC circuit and for any AC circuit feeding a purely resistive load.

• Watts are real power: the rate at which a device turns electricity into heat, light or motion. It is the appliance label figure.
• Volts are electrical pressure. You supply this term, and it belongs to the circuit rather than to the appliance.
• Amps are the rate of flow of current. Everything protective is rated in amps: breakers, fuses, wire gauge, DC sockets, charge controllers and battery management systems.

That is why the conversion exists. Manufacturers state consumption in watts and survivability in amps. Nothing is decided until you translate one into the other.

Watts to amps conversion table

These cover the two AC voltages in a US home and the 12V DC bus in a car, van or portable battery. They assume a power factor of 1, which holds for heaters, kettles and filament lighting; motors need the correction below.

The 1,500W row is marked because it causes the most trouble. A space heater draws 12.5A on a 120V circuit: inside the 15A rating of a standard breaker, but outside the 12A it may carry continuously. Hence one heater per circuit.

Battery-side amps at 12V, 24V and 48V

Battery banks are built at three nominal voltages, and the same load produces very different current on each. These are ideal figures; for an AC load, divide again by an inverter efficiency of about 0.9.

For a single lithium cell the nominal figure is 3.7V, so a cell delivering 10W passes 2.7A and one delivering 50W passes 13.5A. Cell voltage falls from about 4.2V charged to about 3.0V empty, so late in a discharge the current runs higher.

Why the amp figure decides your kit

A power station is advertised in watts and watt-hours, but almost every limit inside it is a current limit. Converting tells you which one you will hit first.

• The 12V DC socket. The car-style socket on a portable battery is usually fused at 10A, sometimes 15A: 120W or 180W, no matter how large the battery behind it. A 2,000Wh unit runs a 12V fridge and a fan from that port and refuses a 12V kettle.
• The solar charge controller. MPPT controllers are rated by the current they deliver to the battery, not by panel wattage. A 40A controller passes roughly 480W of array on a 12V bank and roughly 960W on a 24V bank: identical hardware, twice the solar, because the voltage doubled. Our solar sizing guide works that through.
• The inverter’s DC input. A 3,000W inverter at full output on a 12V bank draws 250A in theory and closer to 280A after 90% conversion efficiency. At 48V the same load draws about 69A. Cable and fuses at 280A are unforgiving, which is why large inverters are rarely 12V.
• The battery management system. Every lithium pack has a maximum continuous discharge current. Exceeding it does not blow anything up; the BMS just cuts out, usually as a compressor starts.

The amps to watts calculator reverses the sum. To size the battery rather than the wiring, the power station finder filters by continuous output and capacity, and the comparison table ranks whole units side by side.

AC, DC and power factor

On DC the conversion is exact, because voltage and current are steady. On AC they oscillate, and for some loads they stop rising and falling in step. That misalignment is power factor, and the formula becomes amps = watts ÷ (volts × power factor).

Power factor sits between 0 and 1. For anything essentially a resistor — heaters, kettles, ovens, hair dryers — it is effectively 1 and can be ignored. For motors, compressors, pumps and cheap LED drivers it runs about 0.6 to 0.9, and ignoring it understates the current.

A 500W motor at a power factor of 0.8 on a 120V circuit draws 5.21A, not the 4.17A the simple sum gives. A 1,200W load at 0.85 draws 11.76A, not 10A. The error is modest on one appliance and compounds across a circuit.

Two consequences follow. An inverter rated at 2,000W may not carry a 2,000W motor load, because inverter ratings assume a power factor near 1. And motors surge at startup, briefly drawing three to seven times their running current, so what a breaker sees at switch-on exceeds anything in the tables. The refrigerator wattage article covers that surge.

Continuous load and the 80% rule

A breaker’s printed rating is not a budget you may spend in full. For loads running three hours or more the limit is 80% of the rating, because sustained current heats the breaker and conductor until it trips on temperature, not on fault.

Those figures are per circuit, not per outlet, and everything on the circuit counts. The 1,440W ceiling on an ordinary 15A circuit explains most nuisance tripping in kitchens and workshops: two 1,000W appliances is 2,000W, which the breaker tolerates for a while and then does not.

The same derating suits a portable battery. A unit rated 2,000W continuous will hold 2,000W, but running it at 1,900W for hours keeps the inverter hot and the fans at full speed, and some units throttle their output. Plan around 80% of the rating. Our sizing guide starts from the load list, not the sticker.

Frequently asked questions