AC Power Calculator

Real, reactive and apparent power for 1-phase or 3-phase loads, plus PF-correction capacitor.

What it does

AC loads such as motors draw current that is out of step with the voltage. That splits power into real (W), reactive (var) and apparent (VA). The calculator also finds the capacitor needed to improve the power factor.

Where it's used

Motors and pumps, sizing generators and UPS units (rated in VA), and power-factor correction.

How to use it

Choose single- or three-phase.
Enter voltage (line-to-line for three-phase), current and power factor.
Enter the supply frequency.
Enter a target power factor to size the correction capacitor.

The formula explained

S: apparent power, VA — what cables and UPS units must carry
P: real power, W — what does useful work and is billed
Q: reactive power, var — circulates back and forth
cos φ: power factor = P / S
C: capacitor that cancels the extra reactive power

Worked example

A three-phase motor at 415 V drawing 25 A with a power factor of 0.75.

Inputs:

System: Three-phase (line-to-line V)
Voltage (RMS): 415 V
Current (RMS): 25 A
Power factor cos φ: 0.75
Supply frequency: 50 Hz
Target power factor: 0.95

The calculator gives:

Real power P: 13.48 kW
Apparent power S: 17.97 kVA
Reactive power Q: 11.89 kvar

Tips and common mistakes

UPS and generator ratings are in VA (S), not W.
Use the line-to-line voltage for three-phase systems.
Correcting much beyond about 0.95–0.98 gives little benefit and risks a leading power factor.

Questions people ask

What does the power factor mean?

Power factor is real power divided by apparent power, P / S = cos φ. At 0.75 only 75 % of the current drawn does useful work; the rest flows back and forth to build the motor’s magnetic field.

How is the correction capacitor sized?

The capacitors must supply Q_c = P × (tan φ1 − tan φ2) of reactive power. For the example motor (13.48 kW, from 0.75 to 0.95) that is about 7.46 kvar, which the calculator turns into microfarads per phase.

Why does a low power factor cost money?

For the same real power, a low power factor means more current, so cables, transformers and switchgear must be bigger and losses are higher. Many electricity tariffs for businesses charge extra when the power factor falls below about 0.9.

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