How this calculator works
Watts are volts times amps, so amps are watts divided by volts. For DC and for AC loads with a power factor of 1 (heaters, incandescent lamps) that is all there is to it. For AC loads with a lower power factor the same watts need more current, so divide by the power factor too. In a three-phase circuit the three wires share the load, which brings in a factor of √3 (1.732) with the line-to-line voltage.
Worked example: a 1,500 W heater on a 120 V circuit draws 12.50 A, which is 1,500 VA, and needs a 20 A breaker if it runs continuously. On 240 V the same heater draws 6.25 A. A 600 W load on a 12 V battery draws 50 A. A 30 kW three-phase load at 480 V and a power factor of 0.85 draws 42.5 A, and a 1,000 W motor load at a power factor of 0.8 on 120 V draws 10.42 A.
Watts to amps at common voltages
Single-phase AC at a power factor of 1, and 12 V DC.
| Watts | 12 V DC | 120 V | 230 V | 240 V |
|---|---|---|---|---|
| 100 W | 8.33 A | 0.83 A | 0.43 A | 0.42 A |
| 500 W | 41.67 A | 4.17 A | 2.17 A | 2.08 A |
| 750 W | 62.50 A | 6.25 A | 3.26 A | 3.13 A |
| 1000 W | 83.33 A | 8.33 A | 4.35 A | 4.17 A |
| 1500 W | 125.00 A | 12.50 A | 6.52 A | 6.25 A |
| 1800 W | 150.00 A | 15.00 A | 7.83 A | 7.50 A |
| 2400 W | 200.00 A | 20.00 A | 10.43 A | 10.00 A |
| 3000 W | 250.00 A | 25.00 A | 13.04 A | 12.50 A |
| 4000 W | 333.33 A | 33.33 A | 17.39 A | 16.67 A |
| 5000 W | 416.67 A | 41.67 A | 21.74 A | 20.83 A |
Tips before you buy
- A 15 amp household circuit at 120 volts can supply 1,800 watts, and 1,440 watts for a load that runs continuously. A 1,500 watt space heater alone is over the continuous limit.
- Higher voltage means lower current for the same power: a 3,000 watt load takes 25 amps at 120 volts and only 12.5 at 240 volts.
- For motors and compressors use the power factor on the nameplate, or 0.8 to 0.9. Starting current is several times the running current.
- For three-phase loads enter the line-to-line voltage (208, 400 or 480 volts), and the total watts of all three phases.
- Check the label on the appliance, which often gives the amps as well as the watts.
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How do you convert watts to amps?
Divide the watts by the volts. 1,500 W ÷ 120 V = 12.5 A. For AC with a power factor below 1, divide also by the power factor.
How many amps is 1,500 watts?
12.5 amps at 120 volts, and 6.25 amps at 240 volts. That is why a 1,500 W space heater nearly fills a 15 amp, 120 V circuit.
How many amps is 1,000 watts?
8.33 amps at 120 volts, and 4.17 amps at 240 volts, with a power factor of 1.
How many amps is 5,000 watts?
41.7 amps at 120 volts, or 20.8 amps at 240 volts.
How do you convert watts to amps for three-phase?
Divide by 1.732 times the line-to-line voltage times the power factor. 30,000 W at 480 V and 0.85 power factor is 42.5 A.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.