Watts to Amps Calculator

How to convert watts to amps

Watts measure the power a load uses. Amps measure the current that flows to deliver it, and volts are the push behind that current. On a direct current (DC) circuit, such as a 12 V battery system, one plain rule links the three: watts = volts × amps. Turn it around and amps = watts ÷ volts.

Alternating current (AC) adds one more number, the power factor. A heater turns all the current it draws into heat, so its power factor is 1 and the DC rule works as it is. A motor also draws current that builds its magnetic field and then flows back each cycle. That current does no work, but the wires still carry it, so the motor draws more amps than its watts alone suggest. The power factor is the share of the current that does real work, from 0 to 1.

• DC: amps = watts ÷ volts
• Single phase AC: amps = watts ÷ (volts × power factor)
• Three phase AC: amps = watts ÷ (1.732 × volts × power factor), with the voltage measured line to line

The calculator runs these formulas in three directions: watts to amps, amps to watts, and watts and amps to volts. It prints each formula with your numbers in it, so you can check the working by hand or copy it into your notes.

Which power factor to use

If you don’t know the power factor, start from the type of load:

• Heaters, oven elements, kettles and incandescent lamps: 1. They are purely resistive.
• Induction motors at full load (fans, pumps, compressors): about 0.85. Published figures run from 0.84 for small motors to 0.91 for large ones.
• Lightly loaded motors: lower, about 0.7 at half load. A motor that idles most of the time draws more current for each watt than its full-load figure suggests.
• LED drivers and electronics: these vary from product to product. Use the power factor or the amps printed on the label.

The best number is on the nameplate. If the label gives amps, use those: they come from testing the actual product, and no formula beats that.

Three phase: where 1.732 comes from

A three-phase supply has three live wires whose voltages peak a third of a cycle apart. The voltage between two of those wires, line to line, is √3 (about 1.732) times the voltage from one wire to neutral. That is why a 208Y/120 V system reads 120 V from a line to neutral and 208 V between lines, and a 480Y/277 V system reads 277 V and 480 V.

The three-phase formula uses the line-to-line voltage. For a balanced load you can also start from the line-to-neutral voltage, amps = watts ÷ (3 × volts × power factor), and get the same answer. Either way, the result is the current in each line, not the total of all three.

This is also why the calculator offers only 208, 240 and 480 V under three phase. 120 V and 277 V are line-to-neutral figures, and putting them in the line-to-line formula gives the wrong amps. Pick Other to type a different line-to-line voltage.

Watts, volt-amps and kilowatts

Volt-amps (VA) are volts times amps, times 1.732 for three phase. They measure the apparent power the wiring has to carry. Watts are the real power the load turns into heat, light or motion. The power factor links the two: kW = kVA × power factor.

At a power factor of 1 the two are equal. Below 1, the calculator shows the apparent power in kVA next to the answer, because generators, transformers and UPS units are rated in kVA as well as kW. A generator that looks big enough in watts can still be too small in kVA for a motor load.

What the answer is for

The amps here are the current the load draws. They are the first number you need when you pick a wire, a breaker or a generator, but they are not the whole answer.

A load that runs 3 hours or more is a continuous load. It is usually counted at 125% when choosing the wire and breaker (NEC 210.19(A)(1) and 210.20(A)). Seen from the other side, a continuous load should stay at or under 80% of the circuit: 12 A on a 15 A circuit, 16 A on a 20 A circuit. The calculator points out a single-phase 120 V result above those figures. Wire size also depends on the length of the run, the insulation and the terminals, and the rules change with the code edition. Your local inspector, the authority having jurisdiction (AHJ), has the final say.

Common mistakes

• Leaving out the power factor for a motor. Watts ÷ volts understates a motor’s current. Divide by the power factor too, or use the nameplate amps.
• Putting line-to-neutral volts in the three-phase formula. 277 V in place of 480 V makes the amps look 1.732 times too high.
• Adding up three-phase amps. The answer is already the current per line. Don’t multiply it by three.
• Reading kW as kVA. A motor rated 10 kW at a power factor below 1 needs more than 10 kVA of supply.

Worked example

1. Inputs: 1,500 W resistive load, single phase AC at 120 V, power factor 1.
2. Amps = watts ÷ (volts × power factor) = 1,500 W ÷ (120 V × 1) = 12.5 A

Questions people ask

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