Amps to watts converter
Converting amps to watts multiplies a current by its voltage, and for AC circuits by the power factor, to find how much real power the circuit delivers. It answers questions such as how much load a 20 A circuit can carry or what a clamp-meter reading means in watts.
Convert amps to watts
Updates as the values change. The page address keeps the values for sharing.
The conversion runs the other way from the usual sizing problem: it starts from a measured or rated current. An electrician reading 9 A on a pump circuit, or a homeowner checking what a generator outlet can supply, needs the wattage that current represents at the circuit's voltage.
Formulas
DC
A 12 V or 48 V DC circuit delivers volts times amps in watts, with nothing to correct for, as long as the voltage is measured at the load.
Single phase AC
Without the power factor, volts times amps gives volt-amperes (VA), the apparent power. Real power in watts is lower whenever the load includes motors or electronic supplies.
Three phase AC
The current here is the reading on one line conductor of a balanced load. Multiplying it by three instead of √3 with the line-to-line voltage overstates the power by about 73%.
I is the current in amperes, V the circuit voltage and PF the power factor. The converter returns watts and kilowatts; for volt-amperes the power factor is set to 1.
Worked examples
What a 20 A, 120 V circuit can carry
The breaker rating stands in for the current, at power factor 1.
P = V × I × PFP = 120 V × 20 A × 1 = 2,400 W
The circuit's full rating corresponds to 2,400 W. A load running three hours or more is limited to 80% of a standard breaker's rating, 1,920 W, by NEC 210.20(A).
A clamp reading of 9.2 A on a 240 V well pump, PF 0.85
The clamp meter reads current, not power; the pump's nameplate gives the power factor.
P = V × I × PFP = 240 V × 9.2 A × 0.85 = 1,876.8 W
The pump uses about 1,877 W of real power. Volts times amps alone would suggest 2,208 VA, which includes reactive current that does no work.
22 A per line on a 480 V three-phase machine, PF 0.88
A balanced three-phase load: the same current is read on each line.
P = √3 × V(L-L) × I × PFP = 1.732 × 480 V × 22 A × 0.88 = 16,095.6 W
The machine draws 16.10 kW. Readings that differ from line to line point to an unbalanced load, for which each phase is converted separately.
Conversion table
| Amps (A) | 120 V single phase (W) | 240 V single phase (W) | 208 V three phase (W) | 480 V three phase (W) |
|---|---|---|---|---|
| 1 | 120 | 240 | 360 | 831 |
| 2 | 240 | 480 | 721 | 1,663 |
| 5 | 600 | 1,200 | 1,801 | 4,157 |
| 8 | 960 | 1,920 | 2,882 | 6,651 |
| 10 | 1,200 | 2,400 | 3,603 | 8,314 |
| 12 | 1,440 | 2,880 | 4,323 | 9,977 |
| 15 | 1,800 | 3,600 | 5,404 | 12,471 |
| 16 | 1,920 | 3,840 | 5,764 | 13,302 |
| 20 | 2,400 | 4,800 | 7,205 | 16,628 |
| 24 | 2,880 | 5,760 | 8,646 | 19,953 |
| 25 | 3,000 | 6,000 | 9,007 | 20,785 |
| 30 | 3,600 | 7,200 | 10,808 | 24,942 |
| 40 | 4,800 | 9,600 | 14,411 | 33,255 |
| 50 | 6,000 | 12,000 | 18,013 | 41,569 |
| 60 | 7,200 | 14,400 | 21,616 | 49,883 |
| 80 | 9,600 | 19,200 | 28,821 | 66,511 |
| 100 | 12,000 | 24,000 | 36,027 | 83,138 |
Generated by the converter's engine. At PF 1 the figures equal volt-amperes; real power for a motor load is the figure multiplied by its power factor.
Common mistakes
Reporting volts × amps as watts for a motor
Volts times amps on an AC motor circuit gives apparent power in VA. Calling that figure watts overstates the real power, and the energy it implies, by the inverse of the power factor.
Treating the breaker rating as the load
A 20 A breaker sets an upper limit; it does not mean 20 A is flowing. Converting the breaker rating gives circuit capacity, while converting a measured current gives the actual load.
Measuring on the neutral of a multiwire circuit
On a multiwire branch circuit the neutral carries only the difference between the two ungrounded conductors. A clamp reading on the neutral says little about the power drawn on either leg.
Multiplying three-phase current by three
With line-to-line voltage the factor is √3, not 3. The factor 3 belongs with line-to-neutral voltage: 3 × V(L-N) × I × PF gives the same answer as √3 × V(L-L) × I × PF.
Frequently asked questions
How many watts is 20 amps?
At power factor 1, 20 A is 2,400 W at 120 V and 4,800 W at 240 V. On a 208 V three-phase circuit it is 7,205 W.
How many watts can a 15 A circuit carry continuously?
A 15 A, 120 V circuit is 1,800 W at its full rating. For a continuous load, NEC 210.20(A) sizes a standard breaker at 125% of the load, which limits that load to 1,440 W.
What is the difference between watts and VA?
VA is volts times amps, the apparent power the wiring must carry. Watts is the real power that becomes heat, light or motion. They are equal only at power factor 1.
Can a clamp meter read watts directly?
A standard clamp meter reads current only. Power clamp meters measure voltage and current together and report watts, VA and power factor; with a current-only meter the power factor has to come from the nameplate.
How are amps converted to watts on an unbalanced three-phase load?
Each phase is converted on its own, using the line-to-neutral voltage, that phase's current and its power factor, and the three results are added. The √3 formula assumes equal currents on all three lines.