watts to amps

I = P ÷ V. Power converts to current only with the voltage: 1,200 W at 120 V is 10 A. For DC and resistive loads the power factor is 1; for motors and transformers it is not, and kVA must be used instead.

The number on a breaker is amps, but the number on every appliance is watts — so the entire electrical trade converts watts to amps at the circuit voltage: I = P/V. A 1,800 W heater on 120 V draws exactly 15 A, which is why a 15 A breaker is the standard US branch circuit. This calculator is the load-calculation workhorse.

By EnginStack Editorial Team | Reviewed against NIST, BIPM and primary-source documentation About →
watts
volts
— amps
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Constants verified against NIST Special Publication 811 (Guide for the Use of the International System of Units) and the 1959 International Yard and Pound Agreement. All values are exact — none are rounded approximations.

watts to amps

Every National Electrical Code load calculation works this way: sum the connected watts, divide by the circuit voltage, and the result tells you the breaker and wire size. At 120 V, 1,800 W is 15 A; at 240 V the same watts need only 7.5 A — which is why high-power appliances run on 240 V. The 80% rule (a continuous load should not exceed 80% of breaker rating) makes this conversion the first step of every circuit design.

amps = watts ÷ volts
I = P/V
Exact watt's law — the ratio of the power and voltage you provide.

Worked Examples

Example 1: Size a circuit for a 1,800 W heater at 120 V

1,800 ÷ 120 = 15 A

Example 2: Size a circuit for 3,600 W at 240 V

3,600 ÷ 240 = 15 A

Example 3: Size a circuit for a 120 W load at 12 V

120 ÷ 12 = 10 A

Common watts to amps Values

wattsvoltsampsContext
1,800 W120 V15 AFull load on a 15 A breaker.
1,440 W120 V12 A80% rule continuous load.
3,600 W240 V15 ADryer on a 240 V circuit.
600 W120 V5 AMicrowave.
120 W12 V10 AAutomotive accessory.

Engineering Context

Watts-to-amps is the most-used electric conversion on this site — it is the inverse of amps to watts, and the breaker-size step in every circuit design. The watts input connects to the energy hub's watts to kW, and the wire-size output connects to AWG to mm². The electric hub collects the full family.

This is the most-used electrical conversion in building work, and it carries one rule that the arithmetic alone will not tell you. A load that runs for three hours or more is treated as continuous, and the protective device must be rated at one hundred and twenty-five percent of the continuous current. A 1,800 watt heater on 120 volts draws 15 amperes, which is a hundred percent of a 15 ampere breaker, so it needs a 20 ampere circuit with 12 AWG conductors even though the division says 15. The same margin applies to the conductors. Sizing a breaker from the converted figure without the continuous-load factor is the single most common wiring error in heating, and it shows up as nuisance tripping before it shows up as anything worse.

More: A to W · V to A · AWG to mm² · Electric Guide · Electric Hub

Related Unit Converters

amps to watts volts to amps AWG to mm² Electric Conversion Guide Electric Converters

Frequently Asked Questions

How do I convert watts to amps?

Divide watts by volts: I = P/V. A 1,800 W heater on 120 V draws 15 A. At 240 V, the same heater draws only 7.5 A.

What is the 80% rule for breakers?

The NEC says a continuous load (running 3+ hours) should not exceed 80% of the breaker rating. So a 15 A breaker handles 12 A continuous = 1,440 W at 120 V. This converter shows both the raw and the derated amps.

How many watts can a 15-amp circuit handle?

At 120 V, 1,800 W at full rating, or 1,440 W continuous per the 80% rule. Multiply the breaker amps by the circuit voltage and you have the watts — the exact inverse of this calculator.