APFC Capacitor Size Calculator

APFC capacitor kVAr formula: kVAr = kW × (tanφ₁ − tanφ₂), where φ₁ and φ₂ are the angles for the current and target power factor — for example, correcting a 100 kW load from PF 0.75 to PF 0.95 needs roughly 55 kVAr of capacitors. An Automatic Power Factor Correction (APFC) panel improves the power factor of an electrical installation by switching in capacitor banks that supply reactive power locally, reducing the reactive (kVAr) component that the utility supply and the upstream transformer would otherwise have to carry. Most industrial and commercial loads — induction motors, transformers, welding sets, and lighting ballasts — are inductive in nature, meaning they draw current that lags behind voltage, and that lag is exactly what a power factor number is measuring.

This calculator works out exactly how much capacitive reactive power, in kVAr, needs to be added to bring a load from its existing, poor power factor up to a desired, improved target power factor. The active power in kW drawn by the load stays constant — correction does not change how much real work is being done — but the reactive power in kVAr drawn from the supply reduces as the power factor improves, which also reduces the total apparent power in kVA that the supply needs to deliver for the same job.

kVA reduction. Beyond the capacitor size itself, this calculator also shows the original and corrected apparent power in kVA side by side, since this is what ultimately determines transformer loading, cable sizing, and utility demand charges. A lower kVA for the same real kW load means headroom is freed up on the upstream supply without adding any additional real load — often enough extra capacity to add a new machine or expand a line without upgrading the transformer.

In practice, this calculation is used by electrical engineers and panel designers to correctly size an APFC panel for a factory, commercial building, or any installation with a poor power factor. Capacitors are commonly available in fixed reference steps rather than continuously variable sizes, and the calculator rounds up to the nearest common bank size — a useful preliminary reference when discussing an APFC panel with a qualified designer or manufacturer, alongside system voltage, frequency, load profile, and harmonic conditions.

kW, kVA, and kVAr — The Power Triangle Explained

Every AC electrical load involves three related quantities that together form what's called the power triangle. kW (kilowatt) is the real, or active, power — the actual useful work the load performs, whether that's spinning a motor shaft, producing heat, or lighting a room. kVA (kilovolt-ampere) is the apparent power — the total power the supply has to deliver, calculated simply as voltage multiplied by current, without regard to how much of it is doing useful work. kVAr (kilovolt-ampere reactive) is the reactive power — the portion of apparent power that doesn't do any useful work but is still drawn from the supply, mostly to build and collapse magnetic fields in motors, transformers, and other inductive equipment.

These three quantities relate to each other geometrically as a right-angled triangle: kW forms the base, kVAr forms the vertical side, and kVA is the hypotenuse. Mathematically, kVA² = kW² + kVAr², which is just the Pythagorean theorem applied to power. The angle between the kVA hypotenuse and the kW base is called φ (phi), and the cosine of that angle, cos(φ), is exactly what we call the power factor. A power factor of 1.0 (or unity) means φ = 0°, meaning there's no reactive component at all and kVA equals kW exactly. A power factor of 0.75 means a significant chunk of the apparent power is reactive rather than real, so the supply has to carry more current than the real work alone would require.

Power factor correction using capacitors works because capacitors are also reactive devices, but their reactive power is exactly opposite in phase to inductive reactive power. When a capacitor bank is connected in parallel with an inductive load, the capacitor's leading reactive power cancels out part of the load's lagging reactive power at the point of connection. The motor or transformer still draws the same reactive power it always needed internally, but that reactive power now circulates locally between the capacitor and the load instead of having to travel all the way back through the cables, meters, and transformer from the utility supply — which is why correction reduces current draw on the supply side without touching the real kW being consumed.

Formula Used for Capacitor kVAr

The standard formula for calculating the required capacitor kVAr to move from an existing power factor to a target power factor is:

The intuition behind this formula follows directly from the power triangle. At the existing power factor, the reactive power the load draws is kW × tanφ₁ (since tanφ equals kVAr divided by kW in the triangle). At the target power factor, the reactive power the load should draw from the supply is kW × tanφ₂. The difference between these two reactive-power values is exactly the amount of reactive power the capacitor bank needs to locally supply, so that the supply only has to provide the smaller, corrected reactive component going forward.

Once the required kVAr is known, this calculator also works out the resulting apparent power before and after correction, using kVA = kW ÷ PF at both the current and target power factor. The difference between these two values — the kVA reduction — is what shows up directly as freed-up transformer, cable, and switchgear capacity.

Worked Example: 100 kW Load, PF 0.75 → 0.95

Take a factory load of 100 kW running at a poor power factor of 0.75, which needs to be corrected to a target power factor of 0.95.

Step 1 — Find the angles. φ₁ = cos⁻¹(0.75) ≈ 41.41°, and φ₂ = cos⁻¹(0.95) ≈ 18.19°.

Step 2 — Find the tangents. tanφ₁ ≈ 0.882, and tanφ₂ ≈ 0.329.

Step 3 — Apply the formula. Capacitor kVAr = 100 × (0.882 − 0.329) ≈ 55.3 kVAr.

Step 4 — Round to the nearest standard size. Since APFC capacitor banks are supplied in fixed steps (5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100 kVAr and larger), the nearest standard size at or above 55.3 kVAr is a 60 kVAr capacitor bank.

Step 5 — Check the kVA reduction. Before correction, kVA = 100 ÷ 0.75 ≈ 133.3 kVA. After correction, kVA = 100 ÷ 0.95 ≈ 105.3 kVA. That's a reduction of roughly 28 kVA for the same 100 kW of real load — capacity that's now freed up on the transformer and incoming cables without adding a single extra kW of actual work.

Why Power Factor Correction Matters

Many commercial and industrial electricity tariffs use power factor, apparent-power demand, or reactive-energy charges to influence the cost of a poor power factor, though the exact mechanism and threshold vary by utility and jurisdiction. Beyond tariff considerations, a poor power factor also means higher current for the same real power, which increases I²R losses in cables and transformer windings, causes larger voltage drops, and can force premature upgrades of transformers, cables, and switchgear that would otherwise have had sufficient headroom.

Correcting power factor with a properly sized APFC panel is one of the more cost-effective electrical improvements available to a facility, since it can eliminate or reduce penalty charges and defer infrastructure upgrades — though the actual financial payback depends strongly on the site's tariff structure, operating hours, load profile, and installation cost. This calculator gives the core sizing number — the required kVAr — that any APFC panel design starts from, before moving on to detailed considerations like step configuration, switching sequence, and harmonic filtering.

How to Use This APFC Calculator

Getting an accurate capacitor sizing only takes three inputs:

1. Enter your connected load in kW — this should be the real, active power drawn by the load or facility, typically available from an energy meter or electricity bill.
2. Enter your current power factor — also usually available from the energy meter, electricity bill, or a power quality analyzer reading. It will be a decimal value less than 1, such as 0.75 or 0.80.
3. Enter your target power factor — the power factor you want to achieve; 0.95 or higher is a commonly used target, though the exact threshold that avoids tariff-related charges depends on your utility and contract. Click Calculate Capacitor kVAr to get the required kVAr, the nearest standard capacitor bank size, and the resulting kVA reduction.

Common Mistakes When Sizing APFC Capacitors

• Using nameplate kW instead of actual metered kW. A motor's nameplate rating is its maximum design output, not what it's actually drawing on average — always size correction around real, metered load data where possible.
• Correcting to unity power factor (1.0). Overcorrection risks pushing the power factor leading rather than lagging during light-load periods, which some tariffs or grid requirements also charge for or limit, and which can cause voltage rise on the bus — most designs target 0.95 to 0.98, not 1.0.
• Ignoring load variation. A fixed capacitor bank sized for full load will overcorrect during low-load periods; this is why real APFC panels use automatic step switching (hence "Automatic" in APFC) rather than a single fixed capacitor.
• Skipping harmonic assessment. Facilities with variable-frequency drives, UPS systems, or other non-linear loads generating harmonics need detuned reactors in series with the capacitors to avoid resonance — this calculator does not account for harmonic filtering.