Power Factor Correction Playbook: Sizing to Payback
The short answer
Power factor correction pays when your utility tariff bills for poor power factor — through kVA billing demand, a power-factor adjustment, or a kVAR charge — or when you need to free up transformer capacity. Size the capacitor bank with kVAR = kW × (tan φ1 − tan φ2), choose an automatic bank if your load varies, and use a detuned (filtered) bank if the facility has variable-frequency drives or other harmonic sources. None of Ohio's investor-owned utilities currently offers business rebates for this work, so payback has to come from bill savings alone.
This page is the project playbook. For what power factor is and the ways Ohio tariffs bill for it, see power factor on Ohio electric bills.
Step 1: Confirm correction will actually pay
Before spending on engineering, answer three questions:
- Does your rate schedule bill for power factor? Look in the tariff and on the bill for "kVA," "kVAR," "power factor" or "reactive." Many tariffs start adjusting below 0.85–0.90. Small commercial schedules often have no provision at all.
- How low is it, and when? Pull 12 months of billed kW, kVA or kVAR and power factor. What matters is power factor during the intervals that set billing demand, not the monthly average.
- What else is on the system? Log a week of data at the main service with a power quality analyzer to capture power factor and harmonic distortion together. The harmonic reading decides what kind of bank you can safely install.
If the tariff has no power-factor provision, correction can still reduce internal losses, improve voltage and free transformer capacity for new equipment — but it won't lower the utility bill directly.
Step 2: Size the bank
The reactive power you need to cancel is:
kVAR required = kW × (tan φ1 − tan φ2), where φ1 = arccos(existing power factor) and φ2 = arccos(target power factor).
Worked example
A plant's billing demand is set at 500 kW with a power factor of 0.78. The target is 0.95.
- tan φ1 = tan(arccos 0.78) = 0.802
- tan φ2 = tan(arccos 0.95) = 0.329
- kVAR = 500 × (0.802 − 0.329) = about 237 kVAR → specify a 250 kVAR bank
Apparent power drops from 500 ÷ 0.78 = 641 kVA to 500 ÷ 0.95 = 526 kVA — about 115 kVA less on the service, the transformer and (on a kVA-billed tariff) the demand charge.
Multipliers (kVAR per kW of load)
| Existing power factor | To reach 0.95 | To reach 0.98 |
|---|---|---|
| 0.70 | 0.692 | 0.817 |
| 0.75 | 0.553 | 0.679 |
| 0.80 | 0.421 | 0.547 |
| 0.85 | 0.291 | 0.417 |
| 0.90 | 0.156 | 0.281 |
Going from 0.95 to 0.98 takes noticeably more kVAR for a small gain; most projects target 0.95–0.97, safely above the tariff threshold.
Size for the load that sets billing demand, then check what happens at light load. A bank sized for full production can push a lightly loaded plant to a leading power factor at night or on weekends, raising voltage — which is why switched banks are the default for variable loads.
Step 3: Choose fixed, automatic or both
| Configuration | Best for | Watch out for |
|---|---|---|
| Fixed capacitor at an individual motor | Large motors that run steadily for long hours | Keep capacitor kVAR below the motor's no-load magnetizing requirement to avoid self-excitation; switch it with the motor |
| Fixed bank at a panel or bus | A steady base of reactive load | Overcorrection when load drops |
| Automatic (switched-step) bank at the main service | Loads that vary by shift, season or process | Controller setup; contactor wear |
| Hybrid | Fixed correction on big steady motors plus an automatic bank for the rest | More pieces to maintain |
In the example above, a 250 kVAR automatic bank in five 50 kVAR steps lets the controller add correction as production ramps up and remove it as the plant empties out.
Step 4: Deal with harmonics before they deal with you
Variable-frequency drives, rectifiers, UPS systems, LED drivers and welders draw non-sinusoidal current. Two consequences:
- Resonance. Plain capacitors combined with the transformer's inductance can resonate near a harmonic frequency and amplify it — the usual cause of blown capacitor fuses, bulging cans and overheating.
- The wrong fix. VFD-fed motors often show a good displacement power factor at the drive input; the problem is harmonic distortion, which capacitors don't solve and can worsen.
If the logger shows meaningful harmonic distortion, specify a detuned bank: each step has a series reactor that tunes it below the fifth harmonic (the dominant one from six-pulse drives), so the bank corrects power factor without resonating. Where distortion itself is the problem, an engineer may recommend harmonic filters and a check against IEEE 519 limits.
Step 5: Run the payback math
The savings depend entirely on how your tariff bills power factor. Two illustrative cases for the 500 kW plant, using an assumed demand rate of $10 per kW or kVA per month and an assumed installed cost of $30,000 for a detuned automatic bank — replace both with your tariff rate and contractor quotes.
| Tariff treatment | Before | After (0.95) | Monthly saving | Annual saving | Simple payback |
|---|---|---|---|---|---|
| Demand billed on kVA | 641 kVA | 526 kVA | 115 × $10 = $1,150 | $13,800 | ~2.2 years |
| kW adjusted up below 0.90 (billing kW = kW × 0.90 ÷ PF) | 577 billing kW | 500 kW (no adjustment) | 77 × $10 = $770 | $9,240 | ~3.2 years |
Adjustment formulas differ by utility and schedule, so rebuild the table from your own tariff language. Don't count on outside funding: AEP Ohio withdrew its business programs in April 2024, PUCO rejected FirstEnergy's in May 2024, Duke kept only low-income programs in 2025, and AES Ohio's ended in 2020. C-PACE financing through participating Ohio communities may be an option for larger projects; confirm eligibility.
Step 6: Commission and maintain
- Verify. Log power factor and harmonics again after start-up, and check the next two bills against the model.
- Inspect annually. Infrared scan of connections, capacitance test of each step (capacitors lose capacitance with age and heat), fuses, contactors, reactor temperature and cabinet ventilation.
- Re-check after changes. Adding VFDs, large motors or an on-site generator can change both the correction needed and the resonance risk.
- Work safely. Capacitors hold a charge after disconnection; maintenance belongs to qualified electricians following arc-flash procedures.
FAQ
How many kVAR do I need to correct my power factor?
Multiply the kW at your billing peak by (tan φ1 − tan φ2) using the existing and target power factors. For 500 kW moving from 0.78 to 0.95, that's about 237 kVAR, so a 250 kVAR bank.
Should I install a fixed or an automatic capacitor bank?
Fixed correction suits large motors that run steadily; an automatic bank suits most facilities whose load varies across shifts, because it avoids overcorrecting at light load.
Can capacitor banks damage equipment?
They can if they resonate with harmonics from drives or other nonlinear loads. A harmonic survey before design, and a detuned bank where distortion is present, prevents most problems.
Are there Ohio utility rebates for power factor correction?
No. None of Ohio's investor-owned utilities currently offers business efficiency rebates, so the project has to pay back from bill savings.