Power Factor Calculator & Capacitor Sizing

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.

UK DNOs require a power factor of at least 0.95 (UK Power Networks customer-responsibilities document 3.17 'Power Factor' states 'no less than 0.95'). On half-hourly metered sites, reactive-power (kVArh) charges within your Distribution Use of System (DUoS) bill are levied on every reactive unit consumed above 33% of active units — the point at which power factor falls below 0.95. The lower the power factor below 0.95, the larger those charges and the greater the risk of running short of supply capacity; there is no separate published surcharge tier at any single figure. 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.

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. Use our Motor Circuit Calculator for motor circuit design including power factor considerations.

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.

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.