Amps to Watts Converter
Amps to watts for DC, single phase AC and three phase AC, using the same power factor handling as the reverse tool.
Enter numbers. Voltage must not be zero and power factor must be between 0.1 and 1.
Reference only. Circuit, breaker and conductor sizing must follow the electrical code that applies where you are, and should be verified by a qualified electrician. This page doesn't give wire gauge, breaker sizing or installation guidance, and a converted figure isn't a substitute for a load calculation.
The formulas
AC single phase: A = W ÷ (V × PF)
AC three phase: A = W ÷ (V × PF × √3)
Each formula multiplies, because you are building power up from the current that carries it. These follow from the definition of the watt as one joule per second and of the ampere as an SI base unit, so they are exact relationships rather than rounded conversion factors. Unit definitions per the NIST Guide to the SI.
Power factor is where a clamp meter and an energy bill part company. Multiply amps by volts on a motor and you get apparent power, measured in volt-amperes, which is larger than the real power in watts that the meter actually bills you for. At a power factor of 0.85 the gap is 15 percent. Set it above rather than letting the tool assume the load is a heater.
Reference table
| Amps | 12 V | 120 V | 240 V |
|---|---|---|---|
| 1 A | 12 W | 120 W | 240 W |
| 5 A | 60 W | 600 W | 1,200 W |
| 10 A | 120 W | 1,200 W | 2,400 W |
| 15 A | 180 W | 1,800 W | 3,600 W |
| 20 A | 240 W | 2,400 W | 4,800 W |
| 30 A | 360 W | 3,600 W | 7,200 W |
| 40 A | 480 W | 4,800 W | 9,600 W |
| 50 A | 600 W | 6,000 W | 12,000 W |
| 100 A | 1,200 W | 12,000 W | 24,000 W |
Every figure above assumes a power factor of 1, so it is the ceiling. A real motor drawing the same current delivers less usable power than the table suggests, never more.
A current reading on its own isn't a power figure
A clamp meter gives you amps. An energy bill and an appliance spec are in watts. Going from the measurement you took to the number you want needs the supply voltage, and that isn't something the meter tells you.
20 amps is 2,400 watts on a 120 volt circuit and 4,800 watts on a 240 volt one. The reading didn't change; the power it represents doubled. Guessing the voltage is the fastest way to be wrong by a factor of two, which is why this tool won't proceed without it.
On a motor or a switched-mode supply the current you measured is partly reactive, so multiplying amps by volts overstates the real power consumed. Set the circuit type above to account for it.
Common questions
What is 20 amps in watts?
At 120 V, 20 amps is 2,400 watts. At 240 V it is 4,800 watts.
How do I convert amps to watts?
Multiply the amps by the volts. For an AC load, multiply by the power factor as well.
Is a 15 amp circuit 1800 watts?
At 120 V, 15 amps works out to 1,800 watts. Continuous loads are normally limited to a fraction of that by electrical code, which is a separate question from the arithmetic and one for a qualified electrician.
Why do I need the voltage?
Because watts measure power and amps measure current, and the relationship between them depends on voltage. Without it the conversion is undefined.
Related converters and guides
Sources
- NIST Guide to the SI, Appendix B: Conversion Factors. National Institute of Standards and Technology. Source for every conversion factor on this page, and for which factors are exact rather than rounded.