Watts to Amps: formula
Enter real power in watts, RMS supply voltage, the applicable power factor as a percent and phase configuration. Use line-to-line voltage for the balanced three-phase calculation. Obtain power factor from measurement or the equipment specification; do not substitute a fixed motor percentage.
A purely resistive ideal load has unity power factor.
Watts to Amps: worked example
| Power (watts) | 1,800 |
|---|---|
| Voltage (volts) | 120 |
| Power factor | 100.0% |
| Supply type | Single-phase or DC |
| Current (amps) | 15 |
| Apparent power (kVA) | 1.8 |
Startup surges of motors draw several times the running current. The tool uses nominal voltage, and real voltage varies. Electrical work should follow code and be done by a qualified person.
Watts on a nameplate may be maximum or rated values. Three-phase systems use line-to-line voltage.
Watts to Amps: changing Power (watts)
| Power (watts) | Current (amps) | Apparent power (kVA) |
|---|---|---|
| 1,440 | 12 | 1.4 |
| 1,620 | 13.5 | 1.6 |
| 1,800 (example) | 15 | 1.8 |
| 1,980 | 16.5 | 2 |
| 2,160 | 18 | 2.2 |
Watts to Amps: compare input ranges
| Input | Tested from | To | Current (amps): low input | Current (amps): high input | Swing |
|---|---|---|---|---|---|
| Supply type | Single-phase or DC | Three-phase | 15 | 8.7 | 6.3 (42%) |
| Voltage (volts) | 108 | 132 | 16.7 | 13.6 | 3 (20%) |
| Power (watts) | 1,620 | 1,980 | 13.5 | 16.5 | 3 (20%) |
| Power factor | 98.0% | 102.0% | 15.3 | 14.7 | 0.6 (4.0%) |
Watts to Amps: individual input comparisons
| Input | Value | Current (amps) | Apparent power (kVA) |
|---|---|---|---|
| Power (watts) | 1,440 | 12 | 1.4 |
| 1,620 | 13.5 | 1.6 | |
| 1,800 | 15 | 1.8 | |
| 1,980 | 16.5 | 2 | |
| 2,160 | 18 | 2.2 | |
| Voltage (volts) | 96 | 18.8 | 1.8 |
| 108 | 16.7 | 1.8 | |
| 120 | 15 | 1.8 | |
| 132 | 13.6 | 1.8 | |
| 144 | 12.5 | 1.8 | |
| Power factor | 96.0% | 15.6 | 1.9 |
| 98.0% | 15.3 | 1.8 | |
| 100.0% | 15 | 1.8 | |
| 102.0% | 14.7 | 1.8 | |
| 104.0% | 14.4 | 1.7 | |
| Supply type | Single-phase or DC | 15 | 1.8 |
| Three-phase | 8.7 | 1.8 |
Watts to Amps: combined input changes
| Power (watts) / Voltage (volts) | 96 | 120 | 144 |
|---|---|---|---|
| 1,440 | 15 | 12 | 10 |
| 1,620 | 16.9 | 13.5 | 11.3 |
| 1,800 | 18.8 | 15 | 12.5 |
| 1,980 | 20.6 | 16.5 | 13.8 |
| 2,160 | 22.5 | 18 | 15 |
Watts to Amps: which assumptions move the result?
Watts to Amps: values and definitions
| Quantity and definition | Value used |
|---|---|
| Power (watts) | 1,800 |
| Voltage (volts) | 120 |
| Power factor — Use 100 for a resistive load such as a heater. Motors are often 80 to 95. | 100.0% |
| Supply type — Single-phase or DC, or three-phase. | Single-phase or DC |
| Current (amps) | 15 |
| Apparent power (kVA) | 1.8 |
Common error: Treating the wattage on a label as the whole story for a motor. Starting current can run several times the running current for a moment.
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Current is power divided by voltage, adjusted for how efficiently the load uses it. A 1,800 watt heater on 120 volts draws 15 amps, right at the limit of a common 15 amp circuit. A motor with a power factor of 85 percent draws more current than its wattage suggests, and a three-phase supply spreads the load so each line carries less.
The apparent power shows what the wiring must handle.
Look for the power factor on a motor's nameplate. Using 100 percent for a motor understates the current and the size of the circuit it needs.
See voltage loss on the voltage drop calculator, Ohm's law and power on the Ohm's law page, and energy units on the power and energy converters. The resistor and LED calculators cover electronics, and the utilities category covers energy use and bills.
Does the current result select a breaker or conductor?
No. For example, 1,440 W divided by 120 V gives 12 A for a resistive load. Circuit protection also requires the applicable load classification, wiring method, equipment ratings and electrical requirements.
How do I convert watts to amps?
Divide watts by volts, and by the power factor if it is less than one. 1,800 watts at 120 volts is 15 amps.
What is power factor?
The ratio of real power to apparent power, 1 for resistive loads and lower for motors and other inductive loads.
How much of a breaker's rating can I use continuously?
Circuit sizing depends on the adopted electrical rules, load classification and listed equipment. A common continuous-load sizing expression uses 125% of that load, but exceptions and equipment ratings matter. A watts-to-amps result alone does not establish conductor size, breaker suitability or permission to add a load.