How is reactive-power compensation calculated?
Start with active power and the existing and target power factors. The simple power triangle provides a preliminary assessment for sinusoidal, steady-state conditions.
The basic equation
Qc = P × (tan φ₁ − tan φ₂). P is active power in kW, Qc is required capacitive reactive power in kvar, and φ₁ and φ₂ correspond to the existing and target power factors. Calculate the angles using φ = arccos(cos φ).
Example: a 100 kW load
For a 100 kW load improving from 0.80 to 0.95 power factor, tan φ₁ = 0.75 and tan φ₂ ≈ 0.329. Thus Qc ≈ 100 × (0.75 − 0.329) = 42.1 kvar. This educational result is not a ready-to-order equipment rating.
Why the equation is not enough
A single reading does not describe a variable load. Stage sizes, light-load overcompensation, harmonics and operating voltage require separate assessment. In distorted systems, total power factor and fundamental displacement cos φ must not be treated as identical.
This example assumes sinusoidal conditions. Final sizing requires measurements and manufacturer technical data.
Schneider Electric — Theoretical principles to improve power factor ↗
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