Power Factor Correction Calculator
Work out the capacitor kVAr that moves a measured load from its present lagging power factor to a target, and what that does to kVA and current. A bill saving appears only if you tell the tool how your tariff bills — none is assumed.
What this tool estimates
The reactive power (kVAr) a correction must supply to move a measured kW from your present lagging power factor to the target, the kVA and current before and after — and a monthly bill effect only from the billing basis and rates you enter.
What it cannot decide
- The bank's nameplate kVAr, voltage rating or steps — step selection is design work.
- Whether harmonics call for a detuned (reactor) bank, or whether a bank could resonate with your transformer.
- Your tariff's power-factor surcharge or rebate, or any saving you have not given it a billing formula for.
- Behaviour at light load: a correction sized at the peak can over-correct when the load drops.
Result
Your bill already averages a power factor at or above the target, so re-billing that month at the target gives a negative kVAh figure. It is shown as it is: correcting to this target would not lower your kVAh.
| Step | Value |
|---|
| Target PF | kVAr | kVA after | Bill effect a month |
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Method version 6 October 2026. A screening estimate, not a design: see the limits below.
How this calculator works — and where it stops
What it calculates
The reactive power (kVAr) a power-factor correction must supply to move a measured real power from its present lagging power factor to a target, the apparent power and line current before and after — and a monthly bill effect only from a billing basis and rates you enter.
Inputs
- Measured real power in kW at the point and moment you want corrected — usually the maximum-demand interval, from a logger, or the bill's maximum demand in kVA × the power factor recorded with it.
- Present lagging power factor at that moment (more than 0, less than 1).
- Target lagging power factor (higher than the present one, at most 1) — your choice; none is recommended.
- Optional, for a bill effect: how your tariff bills maximum demand (kVA, kW or no demand charge) with the demand charge in ₹ per kVA per month and any minimum billable demand; and how it bills energy (kVAh or kWh) with the rate in ₹ per kVAh and one month's kWh and kVAh from the same bill.
Method
- tan φ1 = tan(acos PF1) and tan φ2 = tan(acos PF2): the kVAr drawn per kW at each power factor.
- Reactive power to supply, Qc (kVAr) = kW × (tan φ1 − tan φ2). A target at or below the present power factor is refused — there is no negative bank.
- Apparent power before and after = kW ÷ PF1 and kW ÷ PF2. At the same kW and voltage, line current falls by 1 − PF1 ÷ PF2.
- Demand billed in kVA: monthly effect = demand charge × (billed kVA before − billed kVA after), where billed kVA is the larger of kW ÷ PF and any minimum billable demand you enter. Demand billed in kW, or no demand charge: no change, shown as ₹0.
- Energy billed in kVAh: monthly effect = rate × (the bill's kVAh − the bill's kWh ÷ PF2) — that month's energy re-billed at the target power factor. The figure is signed: a bill that already averages above the target gives a negative number, shown as it is. Energy billed in kWh: no change, shown as ₹0.
- No billing basis entered: no saving is shown at all. The yearly figure is the monthly one × 12, shown beside it, never alone.
- Sensitivity: kVAr, kVA and the monthly effect at targets of 0.90, 0.92, 0.95, 0.97, 0.99 and 1 (those above the present power factor) and at your own target.
Assumptions
- Both power factors are lagging and measured at the same moment as the kW. Correction sized at the maximum-demand interval is right for that interval only; at lighter load the same kVAr over-corrects, which is why automatic (APFC) panels switch in steps.
- Qc is the reactive power delivered at the operating voltage. A capacitor's output varies with the square of its voltage — a unit rated 440 V delivers about 89% of its nameplate kVAr on a 415 V bus, since (415 ÷ 440)² ≈ 0.89 — and series detuning reactors change it again, so the nameplate rating is chosen in design.
- The kVAh effect assumes the correction holds the month's average power factor at the target. How a meter counts leading power factor at light load varies with the metering convention — ask the DISCOM how yours does.
- Any rate you enter is used as given: the tool holds no tariff. Billing basis, demand charges, kVAh billing and power-factor surcharges or rebates are set by your state electricity regulatory commission's tariff order and applied by the DISCOM.
Limitations
- No step selection, nameplate rating or capacitor voltage rating — the result is the reactive power needed before step selection.
- No harmonic study. Plain capacitors can resonate with the supply transformer's inductance and amplify harmonics from drives, UPS systems and rectifiers; whether the bank must be detuned, and what kVAr each detuned step then delivers, needs engineering review.
- One operating point only: no load profile, light-load behaviour, switching transients or controller settings.
- No power-factor surcharge or rebate, and no saving at all unless a billing basis is entered. kW-billed demand and kWh-billed energy do not fall with power factor, and the tool shows that as ₹0 rather than inventing a figure.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Measured real power 100 kW
- Present power factor 0.80, lagging
- Target power factor 0.95, lagging
- Maximum demand billed in kVA, at an illustrative ₹300 per kVA per month — use the rate on your own bill
- Energy billing: not entered
Working
- tan φ1 = tan(acos 0.80) = 0.75; tan φ2 = tan(acos 0.95) = 0.3287.
- Qc = 100 × (0.75 − 0.3287) = 42.13 kVAr.
- Apparent power falls from 100 ÷ 0.80 = 125 kVA to 100 ÷ 0.95 = 105.26 kVA — 19.74 kVA less. At the same kW and voltage, line current falls by 15.8%.
- Demand effect = ₹300 × (125 − 105.26) = ₹5,921 a month; × 12 = ₹71,053 a year, if every month's peak and power factor look like this one.
Result. 42.13 kVAr before step selection, taking 19.74 kVA off the billed demand — ₹5,921 a month at the illustrative ₹300 per kVA. With no billing basis entered, the tool shows the kVAr and the kVA and no saving.
Sensitivity — what moves the answer
- The last hundredths of power factor cost the most kVAr. From 0.80, a target of 0.90 needs 26.57 kVAr, 0.95 needs 42.13, 0.97 needs 49.94, 0.99 needs 60.75 and unity 75 — while the apparent power falls only from 105.26 kVA at 0.95 to 101.01 kVA at 0.99.
- The starting power factor matters as much as the target: correcting the same 100 kW to 0.95 needs 55.32 kVAr from 0.75 but 29.11 kVAr from 0.85.
- kVAr scales directly with kW, so an under-measured peak under-sizes the correction in the same proportion.
- A minimum billable demand can absorb the saving: with a 115 kVA floor in the worked example the demand effect falls from ₹5,921 to ₹3,000 a month, and with a floor above 125 kVA it is nil.
How engineers use the result
To size the order of a correction before a power-quality survey — how many kVAr, and whether the bill effect justifies a study — and to check a quoted APFC rating against the measured load and the target.
When a professional design must replace it
Before ordering a bank or an APFC panel: an electrical engineer logs the load and harmonics at the incomer over a full production cycle, checks for resonance with the transformer, decides between plain and detuned steps, selects the step sizes and the nameplate voltage, and confirms the bill effect against your tariff order and meter.
Sources
- Reactive power to supply: Qc = P × (tan φ1 − tan φ2), from the power triangle (Q = P × tan φ; S = P ÷ cos φ) — definition; no code clause is applied by this tool
- Schneider Electric, Electrical Installation Guide — Theoretical principles to improve power factor — states the same relation, Qc = P (tan φ − tan φ′)
- Schneider Electric, Electrical Installation Guide — Choice of transformer rating — power-factor improvement is one of the considerations in the apparent power a transformer must carry
- Capacitor output and voltage: Q = V² ÷ Xc, so output varies with the square of the applied voltage — definition
- Tariff terms — billing basis, demand charge, kVAh billing, power-factor surcharges and rebates — set by each state electricity regulatory commission's tariff order and applied by the DISCOM; use the figures on your own bill and tariff order — the tool holds none
Questions people ask before using it
What information do I need before sizing power factor correction?
The real power in kW at the moment you want corrected — usually the maximum-demand interval, from a logger or from the bill (if it gives maximum demand in kVA with a power factor, kW = kVA × PF) — the lagging power factor at that moment, and your target. For a bill effect, add how your tariff bills demand (kVA or kW) and energy (kVAh or kWh), with the rates from your bill or your state's tariff order; for kVAh billing, take the month's kWh and kVAh from the same bill.
Which site conditions could change the kVAr result?
A load that swings between shifts — the kVAr needed at light load is different, which is why automatic panels switch in steps; harmonic distortion from drives, UPS systems and rectifiers, which plain capacitors can amplify by resonating with the transformer and which changes what a detuned step delivers; the supply voltage, because a capacitor's output varies with the square of its voltage (one rated 440 V gives about 89% of its nameplate at 415 V); capacitors already installed, or ageing ones that have lost capacitance; and correcting at large motors rather than the main bus.
What should I send an engineer for a project-specific power factor review?
The single-line diagram; twelve months of bills showing maximum demand, kWh, kVAh, power factor and any power-factor surcharge or rebate; details of existing capacitor banks or APFC panels and their condition; a power-quality log at the main incomer over a full production cycle, with harmonics; the transformer rating and impedance; and a list of large drives, UPS systems and other non-linear loads.
Optional: ask an engineer to look at it
The result above is yours with no form. If you want an engineer to check it against your drawings, schedules or bills, send a request. Nothing is sent until you press the button.
Request saved
It reached our team with exactly what the preview showed. An engineer will use the number you gave if they need your drawings or data.
Prefer to talk? WhatsApp or the contact form.
Got your number — what next?
A calculator gives you a first figure. These take it further.
The work itself
Read before you specify
- Factory Power Factor Correction: Read the Bill, Size the kVAr, Check for Resonance
- Maximum Demand and Time-of-Day Tariffs: Plan the Expansion on Interval Data
- LT vs HT Connection for a Factory: Choosing the Supply for an Expansion
- Two Panels, Same Price, Ten Years Apart in Life: Reading an LT Panel Spec
The terms behind it
Delivered work with this scope
- Full MEPF Package for JMH Prime, Jeewan Mala Hospital — Karol Bagh, New Delhi— Jeewan Mala Hospital
- SBJ Nirmal Rotavator-Parts Distribution Warehouse — Ludhiana— SBJ Nirmal Products (SBJ Forging Pvt. Ltd.)
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