Electrical Power Calculator

By Calxy · Calculation methodology

Find real power, current or voltage for a DC, single-phase or three-phase circuit, and see apparent power, reactive power and phase angle from the power factor.

Printed from Calxy · https://www.calxy.net/electrical/electrical-power-calculator

Real power2.0700 kW
V

1e-9 – 10000000

A

1e-9 – 10000000

Between 0 and 1. Resistive loads such as heaters are 1; motors are typically lower. Use the nameplate value when you have it.

Real power

2.0700 kW

  • Real power2,070.0000 W
  • Apparent power2.3000 kVA
  • Reactive power1.0025 kvar
  • Phase angle25.84 °
  • Current10.0000 A
  • Voltage230.0000 V

Assumes a sinusoidal supply and, for three-phase, a balanced load. Power factor here is displacement power factor; harmonics are not modeled.

How this was calculated

Real power P = 230 V × 10 A × 0.9 = 2,070 W.

Apparent power S = 230 V × 10 A = 2,300 VA.

Reactive power Q = √(S² − P²) = √(2,300² − 2,070²) = 1,002.546757 var.

Formulas for DC, single-phase and three-phase power

In a DC circuit, power is simply voltage times current: P = V × I. A 12 V load drawing 5 A uses 60 W.

AC circuits have three related power quantities. Apparent power S (volt-amperes, VA) is voltage times current. Real power P (watts, W) is the part that does work, P = S × PF, where PF is the power factor. Reactive power Q (volt-amperes reactive, var) is the part that flows back and forth to charge motor windings and capacitors: Q = √(S² − P²).

For a balanced three-phase load measured with line-to-line voltage, multiply by √3 (about 1.732): S = √3 × V × I and P = √3 × V × I × PF.

Power formulas by circuit type
CircuitReal power (W)Current (A)
DCP = V × II = P ÷ V
AC single-phaseP = V × I × PFI = P ÷ (V × PF)
AC three-phaseP = √3 × V × I × PFI = P ÷ (√3 × V × PF)

Worked example: kW to amps on a three-phase supply

A 10 kW three-phase motor load runs at 400 V line-to-line with a power factor of 0.85. Current I = 10,000 W ÷ (1.732 × 400 V × 0.85) = 17.0 A.

The apparent power is 10 kW ÷ 0.85 = 11.8 kVA, which is what the supply cable and breaker actually have to carry.

Why kVA and kW differ

Generators, transformers and UPS units are rated in kVA because their heating depends on current, whatever the power factor. The load's useful output is in kW. With a power factor of 1 the two are equal; with 0.8, a 100 kVA transformer can supply only 80 kW.

If you do not know the power factor, a resistive load such as a heater or incandescent lamp is 1. For motors and electronic supplies use the nameplate or datasheet value rather than guessing.

How to use the Electrical Power Calculator

Choose the circuit type and what you know, then enter the values.

  1. Pick the circuit type

    Choose DC, single-phase AC or three-phase AC.

  2. Choose what you know

    Select voltage and current, power and voltage, power and current, or apparent power and voltage.

  3. Enter the power factor

    For AC, enter the power factor from the equipment nameplate, or 1 for resistive loads.

  4. Read the result

    See the answer with real, apparent and reactive power and the working.

References

Frequently asked questions

How many watts is 10 amps?

It depends on the voltage. At 230 V single-phase with a power factor of 1 it is 2,300 W; at 120 V it is 1,200 W. Enter your voltage and power factor to get the exact figure.

What voltage should I enter for three-phase?

Enter the line-to-line voltage, such as 400 V or 480 V. The √3 factor in the formula accounts for the three phases.

Is kVA the same as kW?

Only when the power factor is 1. Otherwise kW = kVA × power factor, so kW is always equal to or less than kVA.

Last updated . Results are estimates for informational purposes only.