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Power Factor Correction Calculator

Find the capacitor size to correct power factor.

Result

55.32 kVAR

How it is calculated

The capacitor must cancel the difference between the reactive power before and after correction:

Qc = P × (tan φ1 − tan φ2), where φ = arccos(power factor)

P is the real (active) power of the load in kW, pf1 is the present power factor and pf2 the target. The answer is in kVAR. Both power factors are decimals above 0 and not more than 1 (0.75, not 75). The target must be higher than the present value, otherwise there is nothing to correct and no result is shown. The result is the capacitor size needed at the load, for an ideal case; it does not select a standard bank size or account for voltage or harmonics.

Worked example

A 100 kW load at power factor 0.75, to be raised to 0.95.

  1. tan(arccos 0.75) ≈ 0.8819
  2. tan(arccos 0.95) ≈ 0.3287
  3. Qc = 100 × (0.8819 − 0.3287) ≈ 55.32 kVAR

A capacitor bank of about 55 kVAR would bring this load to 0.95, which also lowers the current drawn from the supply.

Try this example →

Common mistakes

  • Entering the power factor as a percentage such as 75: it must be a decimal like 0.75, and values above 1 give no result.
  • Entering apparent power (kVA) instead of real power (kW): the formula needs kW.
  • Putting the target in the first box and the present value in the second: the target has to be the larger one, or the tool gives nothing.
  • Aiming for exactly 1.00: the required capacitor grows quickly near unity and over-correction can cause a leading power factor, so utilities and designers usually target a value like 0.95.

About this calculator

This calculator finds the reactive power a correction capacitor must supply, Qc = P·(tanφ₁ − tanφ₂), to raise a system's power factor from its current value (cosφ₁) to a target value (cosφ₂), where P is real power and the tangent terms come from the phase angle between voltage and current in each state. Poor power factor (common in industrial loads with many motors) means more current is drawn than the real power actually used requires, since some current only supports the reactive (non-working) component of the load.

Electrical engineers and plant maintenance staff sizing capacitor banks to avoid utility power-factor penalty charges, and electrical engineering students studying AC power systems use this.

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Comments (1)

  • Editorial team

    The answer is the reactive power to cancel, so the capacitor bank you buy comes in fixed steps and you will pick the nearest size. For 100 kW going from 0.75 to 0.95 the tool shows 55.32 kVAR. Correcting all the way toward 1.0 usually needs far more capacitance for little extra gain. What power factor target is your utility asking for?

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