Watts to Amps Calculator

Going the other way? The amps to watts calculator turns a current reading into watts.

Watts to amps calculator

To convert watts to amps, divide the watts by the voltage and by the power factor. The formula is I = W ÷ (V × PF). A 1,500 W device on a 120 V circuit with a power factor of 1 draws 12.5 amps, and the same device on 240 V draws 6.25 amps.

Enter the watts from the nameplate, the voltage, and the power factor. The calculator returns the current in amps. If you do not know the power factor, leave it at 1 for heaters and lights.

The formula in plain words

Amps = watts ÷ (volts × power factor).

Think of voltage as pressure and amps as flow. Watts is the work done by that flow. For the same work, higher pressure needs less flow. So a higher voltage means fewer amps.

The power factor is 1 for a simple heating element. For motors and some electronics, current and voltage are out of step. Some current does no useful work, so the amps are higher than watts divided by volts alone.

Which formula to use: DC, single-phase, three-phase

System	Formula	Example
DC (battery, solar, 12 V or 24 V)	Amps = W ÷ V	480 W at 24 V = 20 A
AC single-phase (home outlets)	Amps = W ÷ (V × PF)	1,500 W at 120 V, PF 1 = 12.5 A
AC three-phase	Amps = W ÷ (1.732 × V × PF)	10,000 W at 240 V, PF 0.9 = 26.73 A

This calculator uses the single-phase formula. Set the power factor to 1 for DC. For three-phase, 1.732 is the square root of 3, and V is the line-to-line voltage. The answer is the current in each phase wire.

Watts to amps chart: 12 V, 120 V and 240 V

All rows assume a power factor of 1. Divide by your real power factor if it is lower.

Watts	Amps at 12 V	Amps at 120 V	Amps at 240 V
100	8.33	0.83	0.42
500	41.67	4.17	2.08
1,000	83.33	8.33	4.17
1,500	125.00	12.50	6.25
1,800	150.00	15.00	7.50
2,000	166.67	16.67	8.33
3,000	250.00	25.00	12.50
5,000	416.67	41.67	20.83

Notice the pattern. Double the voltage and the amps are cut in half. That is why large loads such as dryers and ranges use 240 V.

Three-phase chart

A 10,000 W (10 kW) load at a power factor of 0.9:

Line voltage	Amps per phase
208 V	30.84
240 V	26.73
480 V	13.36

For 10 kW at 208 V the math is 10,000 ÷ (1.732 × 208 × 0.9) = 10,000 ÷ 324.2 = 30.84 A.

Quick checks for common loads at 120 V

These wattages are examples. Your nameplate is the real number.

Load	Watts	Amps at 120 V
LED bulb	10	0.08
Microwave	1,200	10.00
Hair dryer	1,800	15.00
Window air conditioner	900	7.50
Portable space heater	1,500	12.50

The air conditioner is a motor, so its true current is higher than 7.50 A once you apply a power factor below 1. At a power factor of 0.85, 900 W is 900 ÷ (120 × 0.85) = 8.82 A.

Worked examples

A space heater, 1,500 W on 120 V at power factor 1:

• Amps: 1,500 ÷ (120 × 1) = 12.5 A

The same heater on a 240 V circuit:

• Amps: 1,500 ÷ (240 × 1) = 6.25 A

A 3,000 W load at 240 V and power factor 0.85, such as a motor:

• Amps: 3,000 ÷ (240 × 0.85) = 3,000 ÷ 204 = 14.71 A

A 1,200 W motor at 120 V and power factor 0.8:

• Amps: 1,200 ÷ (120 × 0.8) = 12.5 A

A 1,000 W load on a 12 V battery:

• Amps: 1,000 ÷ 12 = 83.33 A

If that 1,000 W runs through an inverter that is 85% efficient, the battery supplies 1,000 ÷ (12 × 0.85) = 1,000 ÷ 10.2 = 98.04 A. Inverter efficiency varies, so check the spec sheet.

How this relates to breakers

A breaker’s rating is the most current it should carry. Multiplying by voltage gives the watts at the limit.

Breaker	Watts at limit	Continuous load at 80%
15 A at 120 V	1,800 W	1,440 W
20 A at 120 V	2,400 W	1,920 W
30 A at 240 V	7,200 W	5,760 W

The 80% column reflects a widely used rule that a continuous load (running three hours or more) should not exceed 80% of the breaker rating. A 1,500 W heater at 12.5 A on a 15 A breaker is over that line, since 1,440 W is the 80% limit. Rules vary, so check your local code and ask an electrician. The breaker size calculator takes this further.

Where to find the watts

Look at the nameplate on the appliance. It lists watts, or volts and amps, or both. If only amps are listed, use the amps to watts calculator in the other direction. If the label shows VA or kVA instead of watts, the kVA to amps calculator is the better fit.

Treat the label as the maximum draw. A tool that runs under load may pull more than its idle rating.

Common mistakes

• Using the wrong voltage. A 240 V appliance at 120 V will not work properly, and the reverse can damage it.
• Leaving out the power factor on motors and compressors.
• Using the single-phase formula on a three-phase load.
• Adding up the wattage of everything on a circuit and forgetting that the total must stay under the breaker’s rating.
• Treating start-up surge as running load. Motors can draw several times their running current for a moment.
• Ignoring battery and inverter losses on 12 V systems.
• Using an extension cord that is thinner than the load needs.

Related calculators

For the opposite conversion, use the amps to watts calculator. The Ohm’s law calculator relates volts, amps and resistance. The wire size calculator and the voltage drop calculator help once you know the current. The electricity cost calculator turns watts and hours into a bill.

Electrical work can cause fires, shock and injury. Have a licensed electrician size circuits and install wiring.