Power Factor Correction Calculator

This calculator sizes the shunt capacitor bank (in kVAr) that raises a load's power factor from its current value to a target, and shows how far apparent power (kVA) drops as a result. If your utility bills demand on kVA, enter the demand charge to estimate the monthly saving.

Facility managers and electrical engineers use it when a utility bill shows a power factor penalty or a kVA demand charge, or when a transformer or feeder is close to its current limit. Inductive loads — motors, compressors, welders and older fluorescent ballasts — are the usual cause of low power factor.

The result assumes a steady, balanced load and sinusoidal current. It gives the total kVAr for the whole load; how that is split between fixed and automatically switched capacitor steps is a design decision. The three-phase power calculator helps you get the kW and power factor from measured voltage and current.

This follows from the power triangle: reactive power is kW × tan φ, so the capacitors must supply the difference between the reactive power at the old and new angles. The method is the standard one described in IEEE Std 1036 for applying shunt capacitors. The monthly saving assumes demand is billed per kVA; if your tariff penalizes reactive energy (kVArh) or low power factor instead, the saving must be worked out from that tariff. Loads with heavy harmonics, such as variable-frequency drives, need detuned capacitors or filters rather than plain capacitors.

Multiply your load in kW by the factor in the table to get the capacitor kVAr. The 150 kW example uses 0.72 → 0.95, which falls between the 0.70 and 0.75 rows.

• 0.95 lagging is the most common target. Many utilities apply penalties or reactive charges below about 0.90–0.95; UK distribution tariffs, for example, charge for reactive energy beyond roughly the 0.95 equivalent. Check your own tariff for the exact threshold.
• 0.97–0.98 can make sense where transformer or generator capacity is tight, but each extra point costs more kVAr per kVA saved.
• Avoid unity or leading. Over-correction at light load pushes power factor leading, can raise voltage and may itself be penalized.
• Correct at the source where practical. Capacitors fitted at large motors switch with the motor, reducing current in the branch wiring too; a central bank only relieves the supply side.

Capacitors and the supply's inductance can resonate at a harmonic frequency. On sites with many drives, UPS systems or LED drivers, a harmonic survey comes first, and detuned (reactor-protected) banks are the usual fix. Never size a motor-mounted capacitor above the motor's no-load magnetizing kVAr, or the motor can self-excite when switched off. Capacitor banks store charge after disconnection, so installation belongs with a qualified electrician or engineer. A VFD energy savings calculator is useful if you are weighing drives as part of the same upgrade.