Engineering

Power Factor Correction Calculator

Calculate the capacitor bank size needed to improve the power factor of an electrical load. Enter the real power, existing power factor, and target power factor to find the required kVAR.

power-factor-correction
Required capacitor size
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Required kVAR —
Current before —
Current after —
Reactive power before —

What does power factor correction tell you?

Power factor correction tells you how much reactive power is needed to improve an electrical system from its existing power factor to a target value. The required correction is normally expressed in kVAR.

Inductive equipment such as motors and transformers normally draws lagging reactive power. Capacitors can supply some of this reactive power locally. This can improve the power factor and reduce the current drawn from the electrical supply.

Power factor correction formula

For a load with real power P: Qc = P × [tan(cos⁻¹ PF₁) − tan(cos⁻¹ PF₂)] Where: Qc = required capacitor size in kVAR P = real power in kW PF₁ = existing power factor PF₂ = target power factor The power factor angles are: φ₁ = cos⁻¹(PF₁) φ₂ = cos⁻¹(PF₂)

The difference between the existing reactive power and the target reactive power gives the amount of reactive power that the capacitor bank needs to supply.

Worked example

Find the required capacitor size for a 100 kW load. Existing power factor = 0.80 Target power factor = 0.95 Qc = P × [tan(cos⁻¹ 0.80) − tan(cos⁻¹ 0.95)] Qc = 100 × [0.75 − 0.3287] Qc ≈ 42.13 kVAR Answer: approximately 42.13 kVAR

How power factor correction reduces current

For the same real power and voltage, a higher power factor means a lower supply current. This is one of the main benefits of correcting a low power factor.

Three-phase current: I = P × 1000 / (√3 × V × PF) Single-phase current: I = P × 1000 / (V × PF) As power factor increases, the required current decreases.

The calculator shows the estimated current before and after correction so you can see the effect of improving the power factor.

Common mistakes when calculating power factor correction

One common mistake is using kVA instead of kW in the power factor correction formula. The standard correction formula uses the real power of the load.

Another mistake is assuming that every electrical load operates at the same power factor. Motors, transformers, and other inductive loads can have different power factors depending on their operating conditions.

It is also important not to install more correction than the system needs. Excessive correction can produce a leading power factor and may create unwanted electrical conditions.

Common questions

  • Power factor correction improves the power factor of an electrical system by supplying reactive power locally. Capacitors are commonly used to provide this reactive power.
  • The required capacitor size can be calculated using Qc = P × (tan φ₁ − tan φ₂), where P is real power, φ₁ is the existing power factor angle, and φ₂ is the target power factor angle.
  • Capacitors provide leading reactive power that can offset the lagging reactive power drawn by inductive loads such as motors and transformers.
  • Yes. For the same real power and voltage, improving the power factor reduces the current required from the electrical supply.
  • A target power factor between about 0.95 and 0.99 is commonly used in many systems. The correct target depends on the installation and the requirements of the electrical system.
  • Yes. The calculator can estimate the required correction for both single-phase and three-phase systems.