Amps to watts formulas
DC: P = I × V
AC single phase: P = I × V × PF
AC three phase: P = 1.732 × I × V × PF
Apparent power (kVA) is the same calculation without power factor, divided by 1,000. Generators, transformers and UPS units are rated in kVA, so compare both numbers when sizing equipment.
Circuit capacity at 120 V and 240 V
| Breaker | 120 V max | 240 V max | 80% continuous (240 V) |
|---|---|---|---|
| 15 A | 1,800 W | 3,600 W | 2,880 W |
| 20 A | 2,400 W | 4,800 W | 3,840 W |
| 30 A | 3,600 W | 7,200 W | 5,760 W |
| 40 A | 4,800 W | 9,600 W | 7,680 W |
| 50 A | 6,000 W | 12,000 W | 9,600 W |
Going the other way? Use the watts to amps calculator. Then size the conductor with the wire size calculator.
How many watts is 20 amps?
2,400 W at 120 V and 4,800 W at 240 V. For a continuous load, stay at or below 80% of that: 1,920 W at 120 V or 3,840 W at 240 V.
Frequently asked questions
How many watts is 15 amps?
1,800 W at 120 V and 3,600 W at 240 V.
What is the difference between watts and VA?
Watts are real power that does work. VA (or kVA) is voltage times current. They are equal only when the power factor is 1.
How many watts is 30 amps at 240 volts?
7,200 W, or 5,760 W if the load is continuous.
Sources
Reference charts
More electrical calculators
Voltage Drop Calculator
Volts lost, percent drop and load voltage for DC or AC runs.
Wire Size Calculator
Smallest AWG for your amps and distance, AC or DC.
Conduit Fill Calculator
Fill percent and max wires for EMT and PVC.
Watts to Amps Calculator
Convert power to current for any circuit type.
Ground Wire Size Calculator
Ground wire size by breaker rating, per NEC 250.122.
VA to Watts Calculator
VA to watts and back, using power factor.