Amps to Watts Calculator

Enter volts and either amps or watts — get the other instantly for DC, single-phase or three-phase AC, with power factor, kVA, and a quick breaker-size check.

Circuit

Power

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Real power—
Apparent power—
Current—
Min. breaker (80% rule)—
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The formulas

DCW = A × V  ·  A = W ÷ V
AC single-phaseW = A × V × PF  ·  A = W ÷ (V × PF)
AC three-phase (line-to-line)W = √3 × A × VL-L × PF  ·  A = W ÷ (√3 × V × PF)
Apparent powerVA = W ÷ PF  ·  3φ VA = √3 × A × V

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Ohm's law Voltage drop Wattage Wire size Solar panel Solar payback

Converting amps to watts (and back)

Watts measure power, amps measure current, and volts connect them: watts = amps × volts. A 15 A load on a 120 V circuit draws 1,800 W; a 1,800 W space heater on 120 V pulls 15 A. That simple product is exact for DC and for purely resistive AC loads such as heaters, kettles and incandescent bulbs.

Power factor for AC. Motors, transformers, fluorescent ballasts and most electronics draw current that is partly out of phase with the voltage. Only the in-phase part does work, so real power is amps × volts × power factor, a number between 0 and 1. The full product, amps × volts, is apparent power in volt-amps (VA) — the figure that sizes wiring, breakers and transformers. A 10 A motor on 240 V at 0.85 PF uses 2,040 W of real power but the circuit must carry 2,400 VA.

Three-phase adds √3. With three conductors sharing the load, power is √3 (about 1.732) × line current × line-to-line voltage × PF. A three-phase 480 V machine drawing 10 A per line at 0.85 PF consumes 1.732 × 10 × 480 × 0.85 ≈ 7,066 W. Make sure the voltage you enter is line-to-line (208, 400, 480 V), not line-to-neutral (120, 230, 277 V).

Breaker sizing. The US electrical code limits continuous loads to 80% of a breaker's rating, so a load's amps ÷ 0.8 gives the minimum breaker; the calculator rounds up to the next standard size (15, 20, 25, 30, 40, 50, 60…). A 15 A continuous load needs a 20 A breaker with 12 AWG wire, not a 15 A one. Use the wire size calculator for conductor gauge and the voltage drop calculator for long runs.

Frequently asked questions

How many watts is 15 amps?
On a 120 V circuit, 15 A × 120 V = 1,800 W (DC or resistive AC). On 240 V it is 3,600 W. For a motor with 0.85 power factor, multiply by 0.85 for real watts.
How many amps is 1500 watts?
1,500 W ÷ 120 V = 12.5 A on a standard US circuit; on 240 V it is 6.25 A; on 230 V (Europe) 6.5 A. Add power factor for AC motors: 1,500 ÷ (120 × 0.9) = 13.9 A.
What is power factor and what value should I use?
The ratio of real power (W) to apparent power (VA). Use 1.0 for heaters, kettles, ovens and incandescent lighting; 0.9–0.95 for modern LED drivers and computer power supplies; 0.8–0.85 for induction motors and older ballasts. When in doubt, check the nameplate.
Which voltage do I enter for three-phase?
Line-to-line voltage — 208 V, 400 V or 480 V in most systems. If you only know line-to-neutral (120, 230, 277 V), multiply by √3 ≈ 1.732 to get line-to-line.
What size breaker do I need for my watts?
Convert to amps, divide by 0.8 for a continuous load, and round up to the next standard breaker size. 1,800 W on 120 V is 15 A, which needs a 20 A breaker. The calculator does this automatically.
Is this the same as Ohm's law?
Related. Ohm's law links volts, amps and ohms (V = I × R); the power law links watts, volts and amps (P = V × I). This calculator uses the power law with power factor and phase corrections for AC. Use the Ohm's law calculator when resistance is involved.

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