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Calculate active power with alternating current

Enter voltage, current and power factor to find the power in watts. Choose a single-phase calculation or a three-phase calculation with a balanced load.

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The RMS value of the voltage. For three-phase, it is the voltage between two phases.

V

The RMS value of the current.

A

A number above 0 and at most 1. Use 1 for a purely resistive load, for example a heating element.

Voltage and current are RMS values. Three-phase applies to a balanced load with the voltage measured between two phases. The result is not an approval of cable, fuse or installation.

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Worked example
  • Calculate: Single-phase
  • Voltage: 230 V
  • Current: 10 A
  • Power factor (PF): 0.8

Result

Power

1,840 W

Working

  1. P = U × I × PF
    230 V × 10 A × 0.8 = 1,840 W

The power factor must be included in the calculation

With alternating current, the power factor PF is part of the calculation of the active power. PF is a number between 0 and 1. Use the power factor that applies to the load you are calculating. If that information is missing, the power factor is unknown.

Voltage and current must be RMS values. The result is shown as active power in W or kW. The calculation does not in itself approve a cable, fuse or installation.

Single-phase and balanced three-phase power

For single-phase alternating current, P = U × I × PF is used. For three-phase alternating current with a balanced load, P = √3 × U × I × PF is used.

In the three-phase formula, U is the voltage between two phases. So do not use a voltage measured between phase and neutral in this field. The assumption of a balanced load must also be met if the three-phase calculation is to be used.

Example of single-phase power

The page's example has a voltage of 230 V, a current of 10 A and a power factor of 0.8. First 230 × 10 = 2,300. Then the power factor is applied: 2,300 × 0.8 = 1,840 W. The full working is 230 × 10 × 0.8 = 1,840 W.