What is power factor and why does it matter?

Power factor (PF) is the ratio of real power (kW) to apparent power (kVA). A PF of 1.0 means all the power drawn is being used productively. A PF of 0.7 means only 70% is productive — the rest is reactive power (kVAr) that flows back and forth, heating cables and transformers without doing useful work. Poor power factor increases energy bills (many commercial tariffs charge for kVA, not just kW), overloads cables and transformers, and can cause voltage drop issues.

What power factor do I need to achieve?

Most UK DNOs and commercial electricity suppliers expect a power factor of 0.95 or better. Below 0.95, you may incur reactive power charges on your electricity bill. Below 0.85, you may face surcharges or even supply capacity issues. Industrial sites with large motor loads often start at 0.7-0.8 PF and need significant correction. A target of 0.95-0.98 is typical for most commercial and industrial installations.

What causes poor power factor?

The main causes are inductive loads: electric motors (especially when lightly loaded), transformers, fluorescent lighting ballasts, and welding equipment. These loads draw reactive current (lagging power factor). Modern electronic loads like VFDs, computers, and LED drivers can also cause poor power factor, though often with a leading component. Large motor-driven equipment (pumps, compressors, HVAC) is the most common cause in commercial buildings.

How does a capacitor bank correct power factor?

Capacitors provide leading reactive power (kVAr) that cancels out the lagging reactive power drawn by inductive loads. The capacitor bank is connected in parallel with the load, typically at the main switchboard. The required kVAr = kW × (tan(θ1) - tan(θ2)), where θ1 and θ2 are the phase angles before and after correction. Automatic power factor correction (APFC) panels switch capacitor stages in and out as the load varies.

Can I overcorrect power factor?

Yes — overcorrection (leading power factor) is possible if the capacitor bank is too large for the current load. This causes the current to lead the voltage, which can resonate with the supply impedance, cause voltage rise, and damage sensitive equipment. APFC panels prevent this by monitoring the PF and only switching in the capacitor stages needed. Fixed capacitor banks should be sized for the minimum expected inductive load, not the maximum.

TradeCalculator.co.uk

Power Factor Calculator & Capacitor Sizing

Calculate power factor from kW and kVA, determine reactive power (kVAr), and size a capacitor bank to improve power factor to your target value.

Real Power (kW)

Active power from meter or calculation

Apparent Power (kVA)

From supply meter or transformer rating

How We Calculate This

This calculator uses the power triangle relationship to determine reactive power and the capacitor bank size needed to improve power factor.

Power Triangle

PF = kW ÷ kVA = cos(θ)

kVAr = √(kVA² - kW²)

Capacitor kVAr = kW × (tan(θ1) - tan(θ2))

Typical Power Factors

Resistive loads: PF = 1.0 (heaters, kettles)
Fluorescent lighting: PF = 0.5-0.85 (uncorrected)
Induction motors: PF = 0.7-0.9 (depends on loading)
Welding equipment: PF = 0.4-0.6
Target: 0.95+ to avoid reactive charges

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.