Calculating kVAr for Power Factor Correction: A Worksheet

To calculate the kVAr capacitor bank needed to improve power factor from a starting value to a target, multiply your active power (kW) by a tabulated factor. The formula is simple; the engineering nuance is in choosing the target (usually 0.95 or 0.98), accounting for harmonic distortion, and avoiding overcorrection at light load. This guide walks through the calculation, with a worked example from a Kenyan factory and a quick reference table.

The formula

For a load drawing P kilowatts at existing power factor cos φ₁, the kVAr needed to reach target cos φ₂ is:

kVAr = P × (tan φ₁ − tan φ₂)

You can do this on a calculator, but in practice we use a multiplier table.

Quick reference: multiplier table

Multiplier = (tan φ₁ − tan φ₂). Multiply your kW load by the value at the intersection of starting and target PF:

Example: A 200 kW load at PF 0.75 corrected to 0.95 needs 200 × 0.553 = 110 kVAr.

Choosing the target power factor

• 0.90: Minimum acceptable on most utility tariffs to avoid penalty. Don't aim here — install fewer steps, get caught when load varies.
• 0.95: The sweet spot. Eliminates KPLC penalty with comfortable margin, modest capacitor bank size, low overcorrection risk on weekends.
• 0.98: Slightly more capacitive headroom; useful for plants where load varies widely. Watch leading PF at light load.
• Unity (1.00): Almost never targeted. Capacitor bank is large, light-load overcorrection causes leading PF which the utility also penalises.

Worked example — packaging factory in Athi River

Step 1 — Total kVAr needed at peak load: Interpolate the table between PF 0.75 (0.553) and PF 0.80 (0.421): at 0.78, multiplier ≈ 0.474. So 320 × 0.474 = 152 kVAr.

Step 2 — Choose number of steps: Because the load varies widely and is partly VFD-driven, split into 6 stages: 1 × 25 kVAr + 5 × 25 kVAr (or 25 + 25 + 50 + 50 = 150 kVAr in 4 stages for a smaller cabinet).

Step 3 — Detuned reactor selection: 35% VFD load means significant 5th and 7th harmonic content. Use detuned filter reactors tuned to 189 Hz (3.78p) ahead of each capacitor stage to prevent resonance with the 5th harmonic. Without detuning, capacitors fail prematurely from harmonic current.

Step 4 — APFC controller settings: Use a Multispan APFC 112 or similar with delta tan φ target = 0.328 (which corresponds to PF 0.95). Set the C/k ratio per stage, dead-band of 0.10, switching delay of 30 s to avoid hunting.

Step 5 — Light-load behaviour: At 30% load (96 kW) all 6 stages off should give PF ≈ 0.85; first stage on raises to 0.95. The controller alone handles this.

Reading the KPLC bill's power factor penalty

The KPLC bill includes a separate power-factor-penalty line. If your monthly average PF falls below 0.90, KPLC charges:

• A penalty proportional to the kVArh consumed beyond the 90% threshold
• Effective rate currently around KSh 1.00 per kVArh — meaningful on industrial loads
• A 320 kW plant operating at 0.78 PF, 16 hours per day, 26 days per month, accumulates roughly 11,000 kVArh of penalty — a monthly bill of KSh 11,000+ that disappears entirely with a correctly sized APFC bank

When NOT to install a fixed capacitor bank

• Load is mostly already at 0.95+ PF (efficient inverter-driven equipment) — no business case
• VFDs are more than 50% of total load AND there's no detuning — capacitor failure within months
• Very low load utilisation (lighting plants only) — overcorrection risk at night

Bottom line

Sizing an APFC bank is a multiplier-table lookup followed by three engineering judgements: target PF (0.95 is the sweet spot), number of stages (more stages = smoother but more expensive), and whether to detune (yes, if any VFDs are present). The economics are usually compelling — payback typically under 18 months purely from the eliminated KPLC penalty.

See our APFC panel solutions for sized capacitor banks, detuned reactors, and on-site commissioning across Kenya.