Watts to amps calculator
Amps = watts ÷ volts. A 1,500 W heater on 120 V draws 12.5 A. For three-phase power divide by √3 × volts, and for motors also by the power factor.
1 for heaters and lights; motors and electronics 0.8–0.95
- Power
- 1,500 W
- Voltage
- 120 V
- Share of a 15 A circuit at this voltage
- 83%
How it is calculated
- Amps = 1,500 ÷ (120) = 12.5
How to calculate watts to amps
Current is power divided by voltage, with the power factor for motors and √3 for three-phase circuits. The worked numbers start from the calculator's default: 1,500 W on 120 V.
Step 1: single phase or DC
Amps = watts ÷ volts
I = P ÷ V
Example 1,500 W ÷ 120 V = 12.5 A
This holds for heaters, incandescent lamps and other resistive loads, whose power factor is 1.
Step 2: motors and electronics: the power factor
Amps = watts ÷ (volts × power factor)
I = P ÷ (V × PF)
Example 1,500 W ÷ (120 V × 0.8) = 15.63 A
The power factor is on the nameplate or in the manual; a lower power factor means more current for the same watts.
Step 3: three phase
Amps = watts ÷ (√3 × line-to-line volts × power factor)
I = P ÷ (1.732 × V × PF)
Example 1,500 W ÷ (1.732 × 208 V × 1) = 4.16 A on each line
Step 4: amps back to watts
Watts = amps × volts × power factor (× √3 for three phase)
P = I × V × PF
Example 12.5 A × 120 V × 1 = 1,500 W
Watts to amps at 120 V and 240 V
A 1,500 W load draws 12.5 A at 120 V and 6.3 A at 240 V: the same power at twice the voltage takes half the current. Values for single phase at a power factor of 1 (heaters, lamps, resistive loads).
| Watts | Amps at 120 V | Amps at 240 V |
|---|---|---|
| 100 | 0.83 | 0.42 |
| 300 | 2.5 | 1.3 |
| 500 | 4.2 | 2.1 |
| 1,000 | 8.3 | 4.2 |
| 1,500 | 12.5 | 6.3 |
| 2,000 | 16.7 | 8.3 |
| 3,000 | 25 | 12.5 |
| 4,000 | 33.3 | 16.7 |
| 5,000 | 41.7 | 20.8 |
- 120 V
- 240 V
How many amps do household appliances draw?
A refrigerator that uses 540–720 running watts on 120 V draws 4.5–6 A. The table lists the 8 most-searched appliances with their running watts from the documents listed under Sources and the amps from the same formula as the calculator, at a power factor of 1. Every appliance is in the appliance wattage chart.
| Appliance | Running watts | Volts | Amps |
|---|---|---|---|
| Refrigerator | 540–720 | 120 | 4.5–6 |
| TV | 23–155 | 120 | 0.19–1.3 |
| Window Air Conditioner | 440–1,250 | 120 | 3.7–10.4 |
| Microwave | 1,050–1,700 | 120 | 8.8–14.2 |
| Central Air Conditioner | 1,930–5,220 | 240 | 8–21.8 |
| Electric Dryer | 2,800–5,600 | 240 | 11.7–23.3 |
| Light Bulb | 6–100 | 120 | 0.05–0.83 |
| Space Heater | 900–1,500 | 120 | 7.5–12.5 |
How many watts can a 15 or 20 amp circuit carry?
Watts = breaker amps × volts. At its rating, a 15 A, 120 V circuit carries 1,800 W, a 20 A, 120 V circuit 2,400 W and a 30 A, 240 V circuit 7,200 W. That is the limit for every load on the circuit together, not for one appliance, and a load that runs for 3 hours or more must stay further below it: the wire size calculator sizes the breaker for a continuous load.
Why the measured current can differ
The result is the current at the voltage you enter. The voltage at an outlet is rarely exactly the nominal 120 or 240 V, so a clamp meter shows a slightly different reading for the same load.
Nameplate watts are a rating. A thermostat or a low setting runs a heater below it, and appliances with a compressor or motor cycle on and off, so the current changes from minute to minute. A motor also draws more than its running current for a moment when it starts; the appliance pages give the starting watts where the manufacturer publishes them.
Motors and electronics with a power factor below 1 draw more amps than watts ÷ volts. When the nameplate or the manual gives the amps, use that figure rather than a calculated one, and size the circuit for the whole load on it, not for one appliance.