Watts, Volts and Amps Calculator
Convert between watts and amps for DC, single-phase or three-phase AC circuits, with power factor, kVA and horsepower.
- Current
- Real power
- Apparent power
- Reactive power
- Horsepower
- Same load on
- Watts
Saved Loads
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| Load | Circuit | Volts | Amps | Watts | kVA | Remove |
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How to Convert Watts, Volts and Amps
Power in watts is voltage times current. For AC circuits, multiply by the power factor, which accounts for loads like motors where current and voltage don't peak together. Three-phase circuits deliver power on three wires, so multiply by the square root of 3 (about 1.732) and use the line-to-line voltage.
Apparent power, in volt-amperes (VA), is voltage times current without the power factor. Reactive power (VAR) is the part that flows back and forth without doing work: VAR = √(VA² − W²).
Worked Examples
A 1,800 W space heater on a 120 V circuit draws 1,800 ÷ 120 = 15 A. A three-phase motor load drawing 100 A at 480 V with a 0.85 power factor uses 1.732 × 480 × 100 × 0.85 = 70,670 W, or about 70.7 kW, and 1.732 × 480 × 100 = 83.1 kVA.
Amps for Common Loads
| Watts | 120 V | 240 V | 208 V, 3-Phase | 480 V, 3-Phase |
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Power factor 1. Divide by the power factor for motors and other inductive loads.
Tips
Breakers and wire are sized for current, not watts. Under the NEC, a continuous load (three hours or more) shouldn't use more than 80% of a breaker's rating, so a 15 A circuit carries about 12 A continuously. Motor nameplates list full-load amps; use those rather than calculating from horsepower when you have them.