Factory Power Factor Correction: Read the Bill, Size the kVAr, Check for Resonance

Updated 6 Oct 2026 · 8 min read · by Secured Engineers Editorial Team

An engineer in a hard hat holds a tablet behind a yellow floor line in an electrical room, facing a row of closed panels and a capacitor-bank cabinet whose glazed door shows rows of capacitor cans. — Concept illustration made for this page with an AI image tool — not a photograph of a Secured Engineers project.

Power factor (PF) is the ratio of the real power a plant uses (kW) to the apparent power the supply must deliver (kVA). Induction motors, transformers and similar loads draw lagging reactive power (kVAr) to magnetise their windings, so the supply carries more current than the useful work needs. Correcting it — usually with capacitors close to the load — changes three things that matter before an expansion: how much transformer capacity is left, how much contract demand the new load will need, and, depending on your tariff, what the bill charges.

Bill terms: how your tariff treats power factor

Tariffs handle PF in two broad ways, and the bill tells you which one applies:

Energy billed in kVAh. For the same consumption, kVAh = kWh ÷ PF, so a lower PF means more billed units, with no surcharge line at all. Punjab's industrial tariff works this way: PSPCL's FY 2026-27 tariff structure (Commercial Circular 09/2026) bills Small Power, Medium Supply and Large Supply industry with fixed charges per kVA and energy charges per kVAh.
Energy billed in kWh, with a PF clause. Some tariffs bill kWh and add a surcharge below a reference PF, and sometimes an incentive above it, in slabs set by the order.

Haryana's and Uttar Pradesh's commissions set their own terms for each category in their current tariff orders — read the order for your connection rather than a neighbouring state's. In either design, demand-based charges are levied on kVA, so a better PF also lowers the maximum demand the meter records for the same kW.

On the bill or meter	What it tells you	What to do with it
Energy units: kWh, kVAh or both	Which billing design applies	If kVAh is billed, PF sits inside every unit
Monthly average PF (kWh ÷ kVAh)	The PF the DISCOM is effectively billing	Compare month by month; look for seasons and shutdowns
Lagging and leading kVArh, where shown	Whether the meter registers over-correction	Ask the DISCOM how leading kVArh is treated before setting a target PF
Recorded maximum demand (kVA) and contract demand	Headroom against the sanction	Recalculate both at the corrected PF
PF surcharge, rebate or incentive lines	The kWh-tariff design applies	Check the slabs in the current order
Metering point: HT or LT side of your transformer	Whether the transformer's own reactive power is billed	Include it in the kVAr requirement if you are metered on the HT side

On the last row: PSERC's general conditions of tariff in its FY 2024-25 order provide for HT consumers to be metered normally on the HV side of the transformer, so the transformer's magnetising kVAr reaches the bill.

Measured PF, not just billed PF

The bill gives a monthly average. Sizing needs the pattern behind it, so log PF, kW and kVAr at the incomer with a power analyser over a representative production cycle — day and night shifts, start-ups and light-load periods. The Bureau of Energy Efficiency's guide notes that PF can be read directly with a power analyser or worked out from the billing meter's kWh and kVAh. The kWh ÷ kVAh ratio gives the average; the log shows how far it swings, and therefore how much of the correction must switch automatically.

Two more measurements belong in the same exercise: harmonic distortion (see below) and the state of any existing capacitor bank. Dead steps, blown fuses and worn contactors are a common reason a bank exists and PF stays low.

Working out the kVAr requirement

The capacitor rating needed to move a load of P kW from PF₁ to PF₂ is:

kVAr = P × (tan φ₁ − tan φ₂), where cos φ₁ = PF₁ and cos φ₂ = PF₂.

A worked example, with inputs chosen to show the arithmetic: 400 kW at PF 0.85 corrected to 0.98. tan φ₁ = 0.620 and tan φ₂ = 0.203, so the requirement is 400 × 0.417 ≈ 167 kVAr. The apparent power falls from 470.6 kVA to 408.2 kVA — about 62 kVA of transformer and contract-demand headroom recovered for the same production, before any new machine is bought.

Turn that figure into a design rather than a catalogue order:

Fixed and switched parts. A fixed bank sized for full load over-corrects at light load. An automatic power factor control (APFC) relay switches capacitor steps in and out against the measured PF; the BEE guide describes the dead band that stops the relay hunting, and the timing that lets a capacitor discharge fully before it is switched back in.
Step size. Choose steps from the logged swing, so the panel can follow the load without hunting at night.
The transformer. If you are metered on the HT side, the transformer's own magnetising kVAr is billed too, and a small fixed compensation for it is often provided.
Large motors. Capacitors switched with an individual motor must not exceed the motor's magnetising requirement, or the motor can self-excite when it is disconnected — size them from the motor data.
The expansion. Leave panel space and rating for the steps the new load will need, instead of building a bank only for today.
The target. Correcting past unity gives a leading PF; on a meter that counts leading kVArh into kVAh, that costs you again. Set the target with the DISCOM's metering in mind.

The power factor correction calculator runs the kVAr arithmetic on your own figures, and the APFC panel glossary entry explains the hardware. Because a transformer is rated in kVA, PF also sets how much of it the new load will use — the factory electrical spare-capacity calculator shows the effect.

Harmonic resonance review

Capacitors and the inductance of the supply form a resonant circuit. Variable-speed drives, rectifiers, UPS systems, induction furnaces and inverters inject harmonic currents; if the resonant frequency falls near one of those harmonics, currents and voltages are amplified — capacitors overheat and fail early, fuses blow and protection trips. Adding plain capacitors to a plant with a large drive load is the classic way to create the problem.

Measure harmonic distortion at the incomer and the main feeders before choosing the capacitor type.
Where harmonic content is significant, use detuned (reactor-protected) steps, which move the resonance below the dominant harmonics; where it is severe, a filtering study is needed instead.
Check again when the expansion adds drives, or when rooftop solar inverters join the same supply.

Our article on harmonics from VFDs and solar inverters covers the measurements in more detail.

What the measurements show	Correction approach	Check first
Mostly steady motor load, little harmonic content	Automatic panel with standard steps, plus a small fixed part for the transformer	The logged PF swing; the existing bank's condition
Large variation between shifts	Automatic panel with steps sized to the swing	Light-load behaviour; leading PF at night
Significant drives, rectifiers or furnaces	Detuned automatic panel	The harmonic spectrum; resonance with the supply
Severe distortion, or limits applied at the point of supply	Harmonic filtering designed from a study	Measured distortion against any limits your DISCOM applies
A few large motors running long hours	Individual compensation at the motors, plus a central panel	Motor magnetising data, to avoid self-excitation
Rooftop solar on the same supply	A controller that handles reduced or reverse kW at the incomer	PF at the DISCOM meter while solar is generating

A tariff-dependent benefit, not a promised saving

What correction is worth depends entirely on your tariff:

kVAh billing: for the same kWh, billed kVAh falls by 1 − PF₁ ÷ PF₂. Moving from 0.87 to 0.99 (example inputs) cuts billed kVAh by about 12% — on the energy charge only, and only if your meter's kVAh reflects the change.
kWh billing with a PF clause: the benefit is the surcharge avoided, or the incentive earned, under the slabs in your current order.
Demand charges: a lower recorded kVA for the same kW reduces kVA-based charges and frees contract-demand headroom for the expansion — see our guide to contract demand and ToD tariffs.

Against that sit the panel's cost, its losses and its maintenance. Capacitors degrade, so step checks, capacitance measurements and contactor servicing belong in an electrical maintenance contract; a dead step quietly returns the plant to its old PF. Work the benefit out from your own bills and the tariff order in force — no general payback figure applies.

A capacitor panel added to an installation is an addition under the CEA 2023 regulations: it is installed by a licensed electrical contractor (regulation 31), and approved in writing by the Electrical Inspector or self-certified before it is connected, depending on your state's notified voltage (regulation 45(6)). Secured Engineers' electrical infrastructure service designs and installs APFC and detuned panels. More guides are on the Insights page; to have an engineer review correction before an expansion, share the single-line diagram, the equipment schedule and recent demand records through a scope review request.

Sources and how to verify

Bureau of Energy Efficiency — guide book chapter "Basics of energy and its various forms" (power factor, kWh and kVAh, maximum demand) and chapter "Energy efficient technologies in electrical systems" (automatic power factor controllers).
Punjab State Power Corporation Ltd — Commercial Circular 09/2026: tariff structure for FY 2026-27, under the PSERC tariff order of 6 March 2026.
Punjab State Electricity Regulatory Commission — general conditions of tariff, Annexure I to the FY 2024-25 tariff order (condition 11: metering of HT consumers).
Central Electricity Authority — CEA (Measures relating to Safety and Electric Supply) Regulations, 2023, notified 8 June 2023: regulations 31 and 45(6).
Schneider Electric — Electrical Installation Guide: choice of transformer rating (a manufacturer's engineering guide; transformers are rated in kVA).

Confirm locally: the billing basis and PF terms in your current tariff order, how your DISCOM's meter registers leading reactive energy, and your state's notified voltage for self-certification.

Frequently asked

What information do I need before sizing power factor correction for a factory?

Twelve months of bills showing kWh, kVAh, maximum demand and any PF charges; logged PF, kW and kVAr at the incomer over a full production cycle; a harmonic measurement; the single-line diagram with the transformer rating and the metering point; the condition of any existing capacitor bank; and the loads the expansion will add.

Which site conditions could change the size or cost of a PF correction panel?

Harmonic-producing loads such as drives, furnaces and rectifiers, which call for detuned steps or filters; large swings in load between shifts; rooftop solar on the same supply; metering on the HT side of your transformer; the fault level where the panel will sit; space and ventilation for the panel; and the extra steps the expansion will need.

What should I send an engineer to review power factor correction before an expansion?

The single-line diagram, the equipment schedule including the new machines, and recent demand records — bills plus any logged PF and harmonic data. With those, an engineer can size the kVAr, choose standard or detuned steps and check the headroom the correction recovers.

Where do I find power factor on my electricity bill?

Look for kWh and kVAh in the consumption block: kWh divided by kVAh is the average PF the DISCOM is effectively billing. Some bills also print an average PF or show lagging and leading kVArh. If your energy charge is per kVAh, PF is built into every billed unit rather than shown as a separate surcharge.

My capacitor bank exists but PF is still low — why?

Usually dead or degraded capacitor steps, blown fuses, worn contactors, a controller set wrongly, or a bank sized for an older load. Harmonics can also destroy capacitors early. Logging at the incomer with a power analyser and checking each step’s capacitance shows which.

Does rooftop solar affect power factor?

It can lower the PF at the DISCOM meter. Most rooftop inverters run at or near unity power factor, supplying real power (kW), so less kW is drawn from the grid while the plant’s reactive demand is still met by the grid, and the ratio falls. Set the correction with the solar plant in mind, and check how your DISCOM bills the result.

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