SpecCalc Hub
Reactive Power Compensation Calculator
Preliminary calculator for reactive power compensation and kVAr capacitor sizing.
Preliminary estimate only. Not a code-compliance document, not installation approval, not final engineering design. Verify with qualified professional and local codes.
Interactive block
Client-side estimate. SpecCalc Hub does not store entered values.
Formula: Qc = P x (tan(phi1) - tan(phi2))
Reference formula
This simplified formula block is educational and depends on the page status and limitations.
Qc = P x (tan(phi1) - tan(phi2))
Assumptions
- Inputs are user-provided.
- The result is an informational estimate.
Limitations
- Professional verification is required before real-world use.
What this page helps with
Reactive power compensation helps reduce unnecessary current by improving power factor on AC loads. This calculator is meant for a first-pass estimate of kVAr compensation and capacitor-bank context when you already know the active power and the present versus target power factor.
It is useful for industrial boards, commercial feeders and meter-based planning reviews, but it is not a harmonic study, a switching-step design, a commissioning sheet or a final protection review.
Formula or method
The simplified compensation method uses Qc = P x (tan(phi1) - tan(phi2)), where P is active power in kW, phi1 comes from the present power factor and phi2 comes from the target power factor. The result is the preliminary reactive power that the capacitor bank would need to supply in kVAr.
Current and capacitance outputs are supporting estimates only. They assume an idealized AC context and do not model detuned reactors, harmonics, resonance, capacitor tolerances, switching transients or utility-specific control strategy.
How to use
- Enter active power in kW together with the measured or documented present power factor. Do not substitute kVA for kW unless you have already converted the load correctly.
- Choose a realistic target power factor such as 0.90, 0.95 or another value that matches the tariff or engineering objective. Do not assume that correcting to 1.00 is automatically the right answer.
- Read the required kVAr together with the capacitor current and capacitance context, then verify step sizing, harmonic conditions, switching arrangement, protection and local practice before any real equipment decision.
Practical examples
Example 1: 50 kW feeder from 0.75 PF to 0.95 PF
A 50 kW load improving from 0.75 to 0.95 PF needs about 27.7 kVAr in the simplified formula. That is useful for early bank sizing, but the real installation may still need staged steps, detuning and switching review.
Example 2: 15 kW board from 0.82 PF to 0.95 PF
A 15 kW commercial board moving from 0.82 to 0.95 PF needs about 5.5 kVAr in the simplified estimate. That may point to a small staged bank rather than one fixed capacitor, especially when the load varies through the day.
Example 3: when the estimate is not enough
If the installation includes VFDs, welders, UPS systems or a history of harmonics, the kVAr estimate alone is not sufficient. Harmonic spectrum, resonance risk, detuned reactor need and switching strategy must be checked separately.
Common mistakes
- Using kVA as if it were the same as active power in kW.
- Targeting 1.00 PF without checking the utility target, overcorrection risk or load variation.
- Skipping harmonic, resonance, detuning and switching-step review after the first kVAr estimate.
Limitations
- This page is a preliminary reactive power and capacitor-sizing estimate, not a final capacitor-bank design.
- It does not model harmonic distortion, resonance, detuned reactors, inrush, switching transients or utility controller logic.
- Protection, cable loading, capacitor duty, thermal environment and local code requirements must be verified separately.
FAQ
How do I calculate kVAr for power factor correction?
Use the active power together with the difference between the present and target power-factor angles: Qc = P x (tan(phi1) - tan(phi2)). This page performs that first-pass estimate transparently.
Does this page choose a capacitor bank automatically?
No. It estimates the compensation target only. Final bank selection still depends on steps, harmonics, switching, protection, duty rating and site practice.
Should I always correct to 1.00 PF?
No. Many sites target a practical value such as 0.90 or 0.95. Full correction can be unnecessary or undesirable when the load varies and overcorrection becomes possible.
Do harmonics change the answer?
Yes. Harmonics can change the capacitor-bank strategy materially because resonance and detuning may become the dominant design issue even when the basic kVAr number looks reasonable.
Related tools
Last reviewed: 2026-06-20
Factual reference
What is kVAr?
kVAr describes the reactive part of power in an AC system. It affects power factor, current and compensation needs, but it is not the same as useful active load power.
How to calculate capacitor size for power factor correction
The simplified estimate uses active power and the difference between present and target power factor: Qc = P x (tan(phi1) - tan(phi2)). That gives a preliminary kVAr compensation requirement, but it does not replace checks for harmonics, switching steps or protection.
Worked example
If the load is 50 kW, present PF is 0.75 and target PF is 0.95, the simplified estimate gives about 27.7 kVAr of compensation. That is useful for early planning, but it is not a final capacitor-bank selection.
Safety and electrical design disclaimer
This page is not a compliance document, equipment selection sheet or final engineering design. Before real use, verify harmonics, resonance, switching steps, protection, inrush behavior and local rules with a qualified professional.
Graphics
This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.