Reactive Power Compensation: How to Size a Capacitor Bank

Many factories pay power factor penalties every month. Many also run their transformers close to full load, even though the real work output is much lower.

In most cases the cause is the same: too much reactive power. A correctly sized capacitor bank fixes it. This guide shows you how to calculate the capacity step by step, with a worked example you can copy.

Quick Answer: The Capacitor Bank Formula

The basic formula for capacitor bank capacity is:

Qc (kvar) = P (kW) × (tan φ1 − tan φ2)

P is your average active power. tan φ1 comes from your present power factor. tan φ2 comes from your target power factor. The sections below explain each input and show how to avoid the common traps.

Why Reactive Power Compensation Matters

Motors, transformers, welders and lighting ballasts all draw reactive power. It does no useful work, but it still loads cables and transformers.

Lower electricity bills. Many utilities charge a penalty when power factor falls below a set limit, often between 0.90 and 0.95.

More available capacity. Less reactive current means less apparent power. Your existing distribution transformer can carry more real load.

Lower losses. Line current falls, so copper losses in cables and windings fall with the square of the current.

Better voltage. Reactive power flow causes voltage drop. Compensation near the load reduces it.

Key Terms at a Glance

How to Determine Capacitor Bank Capacity: 6 Steps

Step 1: Collect Reliable Load Data

Use real measurements, not nameplate totals. Nameplates show maximum values, and few plants run at full load all day.

There are two common methods.

Method A: Use your utility bills. Take monthly kWh and kvarh. Then tan φ = kvarh ÷ kWh. Use the month with the highest demand.

Method B: Use a power analyzer. Record at the main incoming panel for at least one full working cycle, ideally one week. Note the average and peak kW, kvar and PF.

Step 2: Choose the Target Power Factor

A target of 0.95 is the usual choice. It meets most utility limits without overcompensation. Going to 0.98 or 0.99 needs far more kvar for a small extra gain, so check your tariff before you aim higher.

Step 3: Calculate the Required kvar

Use the formula from above, or multiply P by the factor in this table. It assumes a target of 0.95.

For a target of 0.95, tan φ2 is 0.329. The multiplier is simply tan φ1 minus 0.329.

Step 4: Check It With a Worked Example

A plant has an average load of 500 kW at a power factor of 0.75. The target is 0.95.

Required capacity: 500 × 0.553 = about 277 kvar. A standard 300 kvar bank is the nearest practical size.

Apparent power before: 500 ÷ 0.75 = 667 kVA.

Apparent power after: 500 ÷ 0.95 = 526 kVA.

The plant releases about 141 kVA of transformer capacity. Line current drops by roughly 21 percent. In many cases that is enough to add new load without buying a larger transformer.

Second Example: Checking Transformer Loading

A factory has a 1000 kVA distribution transformer. Measured load is 800 kW at a power factor of 0.80.

Apparent power now: 800 ÷ 0.80 = 1000 kVA. The transformer is fully loaded.

Capacitor size: 800 × 0.421 = about 337 kvar. A 350 kvar bank is the nearest practical size.

Apparent power after: 800 ÷ 0.95 = 842 kVA. The transformer is now at about 84 percent load.

The factory gains about 158 kVA of headroom without changing the transformer.

Step 5: Include the Transformer's Own Reactive Demand

Your transformer is also an inductive load. Its magnetizing current draws reactive power even at no load, typically a small percentage of rated kVA. Its leakage reactance adds more as load rises.

If you measure at the low-voltage side, this is already included. If you calculate from downstream loads only, add a fixed allowance. This applies to both an oil-immersed transformer and a dry type transformer.

Step 6: Split the Bank Into Fixed and Automatic Steps

Loads change during the day. A single large capacitor would overcompensate at light load and push the power factor leading.

The usual solution is an automatic APFC panel with a controller and several steps. Choose a smallest step of about 5 to 10 percent of the total, such as 12.5 or 25 kvar. Use a small fixed bank only for the constant base load, such as transformer no-load demand.

Harmonics: When a Plain Capacitor Bank Is Risky

Variable frequency drives, rectifiers, LED drivers and UPS systems create harmonic currents. A capacitor and the supply inductance can form a resonant circuit. If resonance lands near a harmonic frequency, currents and voltages can rise sharply and damage the bank.

As a common rule of thumb, compare the non-linear load with the transformer rating.

A detuned reactor makes the bank behave inductively above its tuning frequency. At 50 Hz, a 7 percent reactor tunes the circuit near 189 Hz, below the 5th harmonic. A 14 percent reactor tunes near 134 Hz and is used where the 3rd harmonic is strong.

For heavily distorted plants, measure harmonics first. A short power quality study costs far less than a damaged capacitor bank.

Capacitor Bank, Detuned Bank or Static Var Generator?

The right technology depends on the load profile and harmonic level.

Select the Right Voltage and Rating

Capacitor rated voltage should be above the system voltage. On a 400 V system, 440 V units are common. With detuned reactors the voltage across the capacitor rises, so 480 V or 525 V units are often used.

For medium voltage networks, such as 11 kV or 23 kV, banks are built from single-phase capacitor units with series reactors and protection. Rating and protection must be coordinated with the switchgear.

Where to Install the Capacitor Bank

Central compensation with an APFC panel is the most common choice. It is simple, easy to monitor and works well with a single transformer substation.

Protection and Maintenance Basics

A good bank needs protection and regular care.

Protection. Use fuses or breakers sized for capacitor inrush, and fit discharge resistors so stored energy drops quickly after switch-off.

Inspection. Check for swollen cases, oil leakage, loose terminals and hot spots at least once a year.

Measurement. Measure capacitance and current each year. A fall of more than a few percent in capacitance signals aging.

Controller settings. Review the target power factor and step delay after any change in load.

Common Sizing Mistakes

Using nameplate power. It overstates the real average load and leads to an oversized bank.

Ignoring light-load conditions. At night or on weekends, an oversized fixed bank can push the system into leading power factor and raise voltage.

Skipping the harmonic check. This is the leading cause of premature capacitor failure.

Forgetting future load. Leave room for extra steps in the panel if you plan to expand.

Ignoring temperature. Capacitors age faster when hot. Allow enough ventilation in the cabinet.

Checklist Before You Order

Confirm these points with your supplier before placing the order.

Data: measured kW, kvar, PF and a harmonic report.

Target: the power factor required by your utility.

Rating: total kvar, step size, rated voltage and reactor percentage.

Environment: ambient temperature, altitude and enclosure protection class.

Interface: space in the substation and the connection to the main low voltage panel.

Frequently Asked Questions

How many kvar do I need per kVA of transformer?

There is no fixed ratio. As a rough planning figure, many plants end up with a bank between 20 and 40 percent of transformer kVA. Always calculate from measured load.

Can a capacitor bank reduce my transformer size?

It can postpone an upgrade or allow a smaller new unit. Lower apparent power frees capacity on a distribution transformer, as the worked example shows.

Is overcompensation harmful?

Yes. It can raise voltage, cause leading power factor and stress equipment. Automatic control prevents it.

Does reactive power compensation work for solar or generator systems?

Yes, but inverter and generator behavior must be checked. Consult the equipment supplier before adding a capacitor bank.

How Compensation Affects Transformer Selection

Power factor and transformer size are linked. A transformer is rated in kVA, not kW. A low power factor means you need more kVA for the same real load.

When you plan a new project, size the transformer from the corrected power factor, not the raw one. This often lets you choose a smaller oil-immersed transformer or dry type transformer and lower both purchase cost and no-load loss.

For existing plants, compensation is the cheapest way to add capacity. It avoids a new foundation, new cabling and a long outage.

Always keep some margin. Leave 10 to 20 percent of transformer capacity free for future growth and for the short peaks that averages hide.

Size From Measured Data, Then Check Harmonics

The right capacitor bank starts with real measurements. Use the formula, add a margin for the transformer, split the bank into steps, and check harmonics before you buy.

Planning a new substation or upgrading an existing one? We supply oil-immersed transformers, dry type transformers, compact substations and low voltage switchgear. Send us your load data and target power factor, and our engineers will recommend a matching transformer and compensation scheme.