Amps to watts calculator
To convert amps to watts, multiply by the voltage: 15 amps at 120 volts is 1,800 watts. The table gives the power the same 15 A carries at 12 V, 240 V, 208 V three-phase and other supplies.
Notice: For general informational purposes only; not electrical or engineering advice. Work on electrical circuits can cause fire, electric shock or death. Mains-voltage work must comply with the applicable electrical code and be performed by a licensed electrician.
Real power
- Voltage Given.
- 120 V
- Current Given.
- 15 A
- Resistance R = V / I = 120 V / 15 A.
- 8 Ω
- Real power P = V x I = 120 V x 15 A.
- 1,800 W
- Apparent power V x I, before the power factor. Wiring, breakers and transformers carry this, not the watts.
- 1,800 VA
| Supply | Circuit | Power |
|---|---|---|
| 12 V | DC | 180 W |
| 24 V | DC | 360 W |
| 120 V | single-phase | 1,800 W |
| 230 V | single-phase | 3,450 W |
| 240 V | single-phase | 3,600 W |
| 208 V | three-phase | 5,404 W |
| 480 V | three-phase | 12.47 kW |
- Ohm's law holds for resistors, heating elements and wire at a fixed temperature. It does not describe LEDs, diodes, batteries or motors, whose current is not proportional to the voltage across them.
- A conductor's resistance rises as it heats, so the cold resistance a meter reads across a lamp filament or heater element is lower than its resistance in use.
- Single-phase AC with a power factor: real power is V x I x pf. The current a breaker and wire see is set by the apparent power, which is why a low power factor needs bigger wiring for the same watts.
How the Ohm's Law Calculator works
Watts = volts x amps. On DC, and on AC into a resistive load, that is all there is: 15 A at 120 V is 120 x 15 = 1,800 W. The same 15 A at 240 V is 3,600 W, and at 12 V it is 180 W. The current alone does not fix the power; the voltage it flows at does.
What a circuit can supply. A 15 A, 120 V circuit carries at most 1,800 W, and a 20 A one 2,400 W. That is the instantaneous ceiling, not a design figure: electrical codes limit a continuous load on a circuit to less than its breaker rating, so plan a heater or a charger that runs for hours well below these numbers. A 30 A, 240 V circuit, the kind a dryer uses, is 7,200 W at its limit.
On AC with a power factor. Real power is watts = volts x amps x power factor. Volts times amps alone gives the apparent power, in volt-amps (VA). A motor drawing 8 A at 120 V with a power factor of 0.8 uses 120 x 8 x 0.8 = 768 W, though it loads the circuit like a 960 VA heater. Both figures appear in the results: watts are what the meter bills; volt-amps are what the wiring carries.
On three-phase. watts = 1.732 x volts x amps x power factor, with the line-to-line voltage and the current in one line. 20 A at 208 V and a power factor of 1 is 1.732 x 208 x 20 = 7,205 W; at 480 V it is 16,628 W. Leave out the 1.732 and the answer is 42% too low; use the line-to-neutral voltage and it is low by the same factor.
Measuring it yourself. A clamp meter reads current, and multiplying by the outlet voltage gives an upper bound on the power. For anything with a motor or a switching power supply the real watts are lower by the power factor, which a plug-in energy monitor measures directly. To turn watts into a running cost, multiply by the hours used and the price per kilowatt-hour, which the electricity cost calculator does.
Ohm's Law Calculator FAQ
How many watts is 15 amps?
At 120 V, 15 x 120 = 1,800 W. At 240 V it is 3,600 W, and at 12 V DC it is 180 W. Watts depend on the voltage as well as the current.
How many watts can a 20 amp circuit handle?
20 A x 120 V = 2,400 W at the breaker's rating. Continuous loads should be kept well below that, because electrical codes limit a continuous load to less than the full rating of the circuit.
What is the difference between watts and volt-amps?
Volt-amps are volts times amps, the apparent power. Watts are the real power, volt-amps times the power factor. For a heater they are equal; for a motor at a 0.8 power factor, 960 VA is 768 W.
How do I convert three-phase amps to watts?
Watts = 1.732 x line-to-line volts x amps x power factor. 20 A at 208 V and a power factor of 1 is 7,205 W.