Formulas
- Watts = volts × amps
- Amps = watts ÷ volts
- Volts = amps × ohms
- Ohms = volts ÷ amps
- Watts = amps² × ohms = volts² ÷ ohms
Watts for common circuits
| Amps | 120 V | 240 V |
|---|---|---|
| 10 A | 1,200 W | 2,400 W |
| 15 A | 1,800 W | 3,600 W |
| 20 A | 2,400 W | 4,800 W |
| 30 A | 3,600 W | 7,200 W |
| 50 A | 6,000 W | 12,000 W |
To see what that power costs, use the watts to kWh converter.
Watts, volts and amps FAQ
How do I convert amps to watts?
Multiply amps by volts: watts = amps × volts. A 15-amp load on a 120-volt circuit draws 15 × 120 = 1,800 watts.
How do I convert watts to amps?
Divide watts by volts: amps = watts ÷ volts. A 1,500-watt heater on 120 volts draws 1,500 ÷ 120 = 12.5 amps.
What is Ohm’s law?
Volts = amps × ohms. Combined with watts = volts × amps, any two of the four values give you the other two.
What voltage do U.S. homes use?
Most U.S. outlets supply about 120 volts. Large appliances such as clothes dryers, ranges and electric water heaters use 240-volt circuits.