Answer first
What this calculator tells you
Convert watts to amps for a given voltage, power factor and single-phase or three-phase supply. Check the current an appliance or a motor draws before you size a circuit. Formula: Amps = watts ÷ (volts × power factor); three-phase amps = watts ÷ (√3 × volts × power factor). At the worked-example inputs, the current (amps) is 15. Holding every other input steady, moving power (watts) from 1,440 to 2,160 moves the result from 12 to 18.
Transparent method
The formula
Check the current an appliance or a motor draws before you size a circuit.
Worked example
Example inputs
How to interpret the result
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.
At the worked-example inputs the current (amps) is 15. It rises with power (watts) and falls as supply type, voltage (volts) and power factor increase.
These are exact physics and chemistry formulas. Real-world results add tolerances from component quality, temperature and measurement error that this calculator does not model.
Before you rely on it
What to check
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.
The common error
Where people go wrong with watts to amps calculator
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.
Sensitivity evidence
How power (watts) changes the current (amps)
Holding every other input at the worked-example value, moving power (watts) from 1,440 to 2,160 moves the current (amps) from 12 to 18: a spread of 6, or 40% of the worked-example result.
| Power (watts) | Current (amps) | Apparent power (kVA) |
|---|---|---|
| 1,440 | 12 | 1.4 |
| 1,620 | 13.5 | 1.6 |
| 1,800worked example | 15 | 1.8 |
| 1,980 | 16.5 | 2 |
| 2,160 | 18 | 2.2 |
Every input, tested
Which input moves the current (amps) most
Of the 4 inputs, supply type moves the current (amps) most (6.3 across the range tested) and power factor moves it least (0.6).
| Input | Tested from | To | Current (amps) at each end | Swing |
|---|---|---|---|---|
| Supply type | Single-phase or DC | Three-phase | 15 to 8.7 | 6.3 (42%) |
| Voltage (volts) | 108 | 132 | 16.7 to 13.6 | 3 (20%) |
| Power (watts) | 1,620 | 1,980 | 13.5 to 16.5 | 3 (20%) |
| Power factor | 98.0% | 102.0% | 15.3 to 14.7 | 0.6 (4.0%) |
Two variables at once
Current (amps) by power (watts) and voltage (volts)
Across the grid the current (amps) runs from 10 to 22.5. Moving power (watts) from 1,440 to 2,160 shifts it by 6 at the middle column, and moving voltage (volts) from 96 to 144 shifts it by 6.3 at the middle row, so voltage (volts) is the bigger lever here.
| 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 |
The highlighted cell is the worked example: 15.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Power (watts) | 1,800 | Enter the power (watts) used in this calculation. |
| Voltage (volts) | 120 | Enter the voltage (volts) used in this calculation. |
| Power factor | 100.0% | Use 100 for a resistive load such as a heater. Motors are often 80 to 95. |
| Supply type | Single-phase or DC | Single-phase or DC, or three-phase. |
| Current (amps) | 15 | |
| Apparent power (kVA) | 1.8 | |
Inputs, definitions and assumptions
Power (watts)
Enter the power (watts) used in this calculation. The prefilled worked-example value is 1,800.
Voltage (volts)
Enter the voltage (volts) used in this calculation. The prefilled worked-example value is 120.
Power factor
Use 100 for a resistive load such as a heater. Motors are often 80 to 95. The prefilled worked-example value is 100.0%.
Supply type
Single-phase or DC, or three-phase. The prefilled worked-example value is Single-phase or DC.
How to use this calculator
- 1Verify the inputs. Gather power (watts), voltage (volts), power factor and supply type from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the current (amps) at 15. Store your own version of it as Scenario A.
- 3Test one change. Start with supply type, the input with the biggest effect here: switching supply type changes the current (amps): single-phase or DC gives 15; three-phase gives 8.7.
- 4Check the boundary. Read the interpretation boundary above before acting on the result.
People also ask
Frequently asked questions
How do you calculate watts to amps?
Amps = watts ÷ (volts × power factor); three-phase amps = watts ÷ (√3 × volts × power factor). Enter power factor in percent (100 means 100%). At the worked-example inputs the current (amps) is 15.
What does the watts to amps result mean?
Check the current an appliance or a motor draws before you size a circuit. At the worked-example inputs the current (amps) is 15. It rises with power (watts) and falls as supply type, voltage (volts) and power factor increase.
How much does power (watts) change the current (amps)?
Holding every other input at the worked-example value, moving power (watts) from 1,440 to 2,160 moves the current (amps) from 12 to 18, a spread of 6.
What are the limits of this watts to amps calculator?
These are exact physics and chemistry formulas. Real-world results add tolerances from component quality, temperature and measurement error that this calculator does not model. The tables on this page test power (watts) only from 1,440 to 2,160; a value outside that range is not tabulated here.
Which input moves the current (amps) most in the watts to amps calculator?
Ranked by how far each moves the current (amps) across the range tested: supply type (6.3, 42%), voltage (volts) (3, 20%), power (watts) (3, 20%) and power factor (0.6, 4.0%).
How much does voltage (volts) matter in the watts to amps calculator?
The worked example uses 120. Holding every other input at its worked-example value, moving voltage (volts) from 108 to 132 takes the current (amps) from 16.7 to 13.6, a swing of 20% of the worked-example figure.
How much does power factor matter in the watts to amps calculator?
The worked example uses 100.0%. Holding every other input at its worked-example value, moving power factor from 98.0% to 102.0% takes the current (amps) from 15.3 to 14.7, a swing of 4.0% of the worked-example figure.
How much does supply type matter in the watts to amps calculator?
The worked example uses Single-phase or DC. With the other inputs left at the worked example, switching supply type changes the current (amps): single-phase or DC gives 15; three-phase gives 8.7.
Which inputs change the apparent power (kva) in the watts to amps calculator?
At the worked-example inputs it is 1.8. Power (watts) takes it from 1.6 to 2 and power factor takes it from 1.8 to 1.8.
Why is pH a logarithmic scale?
Hydrogen ion concentrations span many orders of magnitude, from about 1 mole per liter in strong acid to 10 to the minus 14 in strong base. A log scale squeezes that into 0 to 14, and each unit is a factor of ten.
Why do parallel resistors total less than any single one?
Each added path gives current another route, so more current flows at the same voltage. More current at the same voltage means lower resistance overall.
Sources and evidence
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