PlantmarginPower factor correction: when it pays, and how to size it
Motors, transformers and welding sets draw reactive power. It does no useful work, but it loads the supply and raises the kVA that sets the demand charge.
How much it costs
Demand is charged on kVA. A plant drawing 400 kW at a power factor of 0.85 draws 470 kVA. At 0.97 the same plant draws 412 kVA.
On a CI2 tariff at USh 700 per kVA, those 58 kVA cost about USh 40,600 a month before VAT. Where a PF surcharge applies, the saving is larger.
Sizing from data
The capacitor rating needed is:
kvar = kW × (tan φ₁ − tan φ₂)
where φ₁ is the angle at the present power factor and φ₂ at the target. For 400 kW from 0.85 to 0.97:
400 × (0.620 − 0.251) = 148 kvar
Choose the next standard bank above that, here 150 kvar. Use an automatic bank with several steps, so that the power factor does not go leading when the load is low.
Use the 90th percentile of measured kW, not the transformer rating. Thirty days of 10-second data shows how the load really moves across shifts.
When to use detuned banks
Sites with many variable speed drives have harmonic currents. These can overheat plain capacitors.
A detuned bank (7% reactors, tuned to about 189 Hz) avoids resonance. Check current distortion (THD-I) before buying. Above about 15–20%, use detuned.
Payback
A 150 kvar automatic bank costs roughly USh 540,000 installed (2026 prices). Against USh 40,000–60,000 a month of avoided charges, the payback is usually 9–15 months.
After installation
Confirm the result on the bill. Plantmargin registers the bank as a measure and compares PF, kVA and cost before and after.