Converting watts to amps requires more than two unit labels. Watts measure power, while amps measure current, so voltage is always part of the conversion. An AC calculation may also need the circuit phase and the load’s power factor. Selecting the formula that matches the electrical system is just as important as entering the numbers correctly.
For a quick result, enter the known values in the watt and ampere calculator. The steps below explain what the calculator does and help you verify the selected inputs.
Information to gather first
Before calculating, identify:
- the real power in watts;
- the operating voltage, not merely a voltage found elsewhere on the system;
- whether the circuit is DC, single-phase AC, or balanced three-phase AC; and
- the equipment power factor for an AC real-power calculation.
Use values that describe the same operating condition. For example, combining maximum rated watts with a nominal voltage and an unrelated measured power factor may produce a mathematically valid number that does not describe the actual load.
DC watts-to-amps formula
For DC, divide real power by voltage:
I = P / V
Here, I is current in amperes, P is power in watts, and V is voltage in volts. A 1,200 W DC load operating at 120 V draws:
I = 1,200 / 120 = 10 A
If voltage decreases while power stays the same, current increases. The same 1,200 W would require 100 A at 12 V. This inverse relationship is a useful reasonableness check.
Single-phase AC watts-to-amps formula
For single-phase AC real power, include power factor:
I = P / (V x PF)
At 1,200 W, 120 V, and a power factor of 0.8:
I = 1,200 / (120 x 0.8) = 12.5 A
At a power factor of 1.0, the same watts and voltage would equal 10 A. A lower power factor therefore produces a higher calculated current when real power and voltage remain fixed. Read why power factor changes AC current for a more detailed comparison.
Balanced three-phase AC formula
For a balanced three-phase circuit using line-to-line voltage:
I = P / (sqrt(3) x V x PF)
For 15,000 W at 400 V and a power factor of 0.9:
I = 15,000 / (1.732 x 400 x 0.9) = 24.06 A
This formula assumes the entered voltage is line-to-line and the load is balanced. A line-to-neutral voltage belongs to a different expression, so confirm the voltage convention before using the result.
Common input and calculation mistakes
Do not leave out power factor merely because a load is expressed in watts. Avoid using the three-phase formula for a single-phase circuit or treating kilowatts as watts; multiply kilowatts by 1,000 first. Also distinguish real power in watts from apparent power in volt-amperes. They are related, but they are not interchangeable when power factor is below one.
You can check the arithmetic by substituting the calculated current back into the corresponding power formula. Small differences are normal when displayed values are rounded. For a foundation in the units themselves, see watts, amps, and volts explained.
Calculation is not installation design
This arithmetic does not determine wire size, breaker size, voltage drop, starting current, equipment suitability, or compliance with an electrical code. Those decisions depend on additional system and installation details. Consult the equipment documentation and appropriately qualified guidance before making electrical design or safety decisions.