Amps to Watts Calculator

Amps and watts measure different quantities, so current alone is not enough. For DC or a single-phase resistive load, P = V x I. A single-phase reactive load also needs power factor. This calculator does not model the square-root-of-three factor used for balanced three-phase power.

Rearrange P = V x I to I = P / V. A 1,500 W space heater on a 120 V outlet draws 1,500 / 120 = 12.5 A; a 1,800 W microwave on the same outlet pulls 15 A, which is the entire 15 A circuit. For 240 V appliances (electric dryer, EV charger), divide by 240 instead: a 7,200 W EV charger pulls 7,200 / 240 = 30 A, which is why it needs a dedicated 40 A breaker (80 percent continuous-load rule). Always size your circuit for the continuous load, not the peak.

Use the appliance instructions and the extension cord's listed amp rating, wire gauge, length, temperature rating, and indoor or outdoor classification. Longer cords can require a larger conductor to limit voltage drop. Do not infer a safe cord from amps alone, and do not exceed either the cord or appliance rating.

On a reactive AC load (motor, transformer, fluorescent ballast), voltage and current fall out of phase, so some current shuttles energy back and forth without doing useful work. Real power is P = V x I x pf, where the power factor pf is between 0 and 1. To deliver the same real watts, a load with a lower pf pulls more amps: an 1,800 W motor at 120 V and pf 0.75 needs I = 1,800 / (120 x 0.75) = 20 A, instead of the 15 A a purely resistive 1,800 W heater would draw. That extra current still has to fit on the breaker, the wire, and the cord, which is why motor circuits are sized for amps, not watts.

At a nominal 120 V, 15 A equals 1,800 VA. A qualifying continuous load is generally limited to 12 A on that circuit, but equipment, wiring, receptacles, shared loads, and code rules still matter. A breaker is not a normal operating control, so have high or continuous loads checked against the installation.