Amps to watts formulas
Watts measure real power, the rate at which energy is used. Amps measure current. You need the voltage to convert one to the other, and for AC you also need the power factor (PF):
DC: W = A × V
AC single phase: W = A × V × PF
AC three phase: W = √3 × A × VL-L × PF ≈ 1.732 × A × V × PF
For three phase, use the line-to-line voltage (208, 240, 480 or 600 V) and the current in one line. For a single-phase 240 V load, use 240 V, not 120 V.
Worked examples
1. A 15 A, 120 V circuit
W = 15 × 120 × 1.0 = 1800 W
That is the theoretical maximum. NEC 210.20(A) and 210.19(A) mean continuous loads (three hours or more) are limited to 80% of a standard breaker rating, so plan for 12 A or 1440 W.
2. A 240 V electric range circuit
A 40 A circuit at 240 V can deliver 40 × 240 = 9600 W, or 9.6 kW. Range nameplates are often 10–12 kW, which is acceptable because NEC Table 220.55 applies demand factors to cooking appliances.
3. A three-phase motor
A motor draws 30 A per line at 480 V with a power factor of 0.9:
W = 1.732 × 30 × 480 × 0.9 = 22,447 W ≈ 22.4 kW
Apparent power is 1.732 × 30 × 480 = 24.9 kVA. Shaft output is lower again because of motor efficiency, which is why a motor drawing about 30 A at 480 V is roughly a 20–25 hp machine. For motor circuits, NEC 430.6 requires conductors sized from the full-load current tables, not the nameplate.
4. A 12 V DC load
A 12 V fridge drawing 4.5 A uses 12 × 4.5 = 54 W while the compressor runs.
Amps to watts reference table (PF = 1)
| Amps | 120 V | 208 V (1φ) | 240 V | 480 V 3φ |
|---|---|---|---|---|
| 1 A | 120 W | 208 W | 240 W | 831 W |
| 5 A | 600 W | 1,040 W | 1,200 W | 4,157 W |
| 10 A | 1,200 W | 2,080 W | 2,400 W | 8,314 W |
| 15 A | 1,800 W | 3,120 W | 3,600 W | 12,471 W |
| 20 A | 2,400 W | 4,160 W | 4,800 W | 16,628 W |
| 30 A | 3,600 W | 6,240 W | 7,200 W | 24,942 W |
| 40 A | 4,800 W | 8,320 W | 9,600 W | 33,255 W |
| 50 A | 6,000 W | 10,400 W | 12,000 W | 41,569 W |
Watts to amps
Reverse the formulas to get current from a nameplate wattage: A = W ÷ (V × PF) for single phase and A = W ÷ (1.732 × V × PF) for three phase. A 5000 W heater at 240 V draws 5000 ÷ 240 = 20.8 A. Because it is continuous, the circuit must be rated at least 125% of that, 26 A, so use a 30 A breaker and 10 AWG copper. Then check the run with the voltage drop calculator.
Watts, VA and power factor
Volt-amperes (VA) are voltage times current with no power factor. Watts are the part of that power that does real work. With motors, transformers and electronic drivers, current and voltage are out of step, so watts are less than VA. Utilities bill in kWh (watts over time), but wires and breakers carry the full current, so size conductors from amps or VA, not watts. For larger loads, the kVA calculator and 3 phase power calculator work in kVA and kW.
Common mistakes
- Using 120 V for a 240 V appliance, which halves the answer.
- Leaving out √3 on three-phase loads, or using it on single-phase ones.
- Assuming a PF of 1 for motors, which overstates real power.
- Using peak voltage instead of RMS; a 120 V outlet is 120 V RMS.