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Amps to Watts Calculator (120 V)

Convert a current draw in amps into watts on a standard US 120 V single-phase circuit.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Power

1800 W

High confidence

P = V × I at 120 V single phase, with a power factor of 1 assumed. Assumes a resistive load. For motors and other inductive loads, multiply by the power factor to get real power.

Conversion factor applied
120 W per A

Working out what a circuit is actually carrying starts with a current figure and a supply voltage, and on a US general-purpose branch circuit that voltage is nominally 120 V. Multiplying the two gives apparent power, and for a heater, kettle or incandescent lamp that is also the real power. The distinction matters as soon as a motor is involved: an inductive load draws current that is partly out of phase with the voltage, so the volt-amps exceed the watts by the reciprocal of the power factor. A compressor pulling 15 A at 120 V presents 1,800 VA to the supply but may only be doing 1,440 W of work. Size conductors and breakers from current, which is what actually heats the cable, and use the wattage for energy and heat-gain calculations rather than for protection.

Add the equipment this sizes

This result is a specification — 1,800 W — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Ohm's law for real power on a resistive single-phase load: P = V x I
  • US residential general-purpose branch circuits are nominally 120 V

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Frequently asked questions

Why does my motor draw more watts than this suggests?
Because an inductive load has a power factor below 1. Volts times amps gives apparent power in VA; real power in watts is that figure multiplied by the power factor, which on a small motor might be 0.8. The two only coincide on resistive loads such as heaters and incandescent lamps.
Can I use my breaker rating as the current?
No. The breaker protects the conductor, and sizing a load calculation from it assumes every circuit is fully loaded all the time. Use the nameplate running current of what is actually connected.
Is US voltage 120 V or 110 V?
Nominally 120 V. The 110 V and 115 V figures are historic and still in casual use, but modern US distribution is specified at 120 V, and calculations should use it. Actual delivered voltage varies with distance from the transformer.