Calculator inputs
Example data
| Case | Inputs | Outputs |
|---|---|---|
| 1 | Single phase, P=5.00 kW, PF=0.80 lagging | S=6.25 kVA, Q=3.75 kvar, φ≈36.87° |
| 2 | Three phase, V=400 V, I=10 A, PF=0.90 lagging | S≈6.93 kVA, P≈6.24 kW, Q≈3.02 kvar |
| 3 | Given P=12 kW and Q=−5 kvar (leading) | S≈13.00 kVA, PF≈0.923, φ≈−22.62° |
Formula used
- S = √(P² + Q²)
- PF = P / S
- φ = atan2(Q, P) (degrees shown)
- Single phase apparent power: S = V × I
- Three phase apparent power: S = √3 × VL-L × I
- Reactive sign: lagging Q > 0, leading Q < 0
How to use this calculator
- Select a calculation mode that matches your known values.
- Choose single or three phase, then enter the required inputs.
- Set reactive direction if Q is being derived.
- Click Calculate to view results above the form.
- Use CSV or PDF downloads to record your outputs.
FAQs
1) What does the power triangle represent?
It relates real power P, reactive power Q, and apparent power S as a right triangle. P is horizontal, Q is vertical, and S is the hypotenuse. The angle φ indicates the phase shift between voltage and current.
2) What is the difference between kW and kVA?
kW measures useful work (real power). kVA is total apparent power delivered by the source. They differ when reactive power is present, because current increases without increasing real work.
3) How do I know if my load is leading or lagging?
Inductive loads (motors, transformers) are typically lagging, meaning Q is positive. Capacitive loads are typically leading, meaning Q is negative. If you enter Q directly, its sign determines the direction.
4) Why must S be greater than or equal to P?
Because apparent power is the vector magnitude of P and Q. By geometry, the hypotenuse cannot be shorter than the adjacent side. If S < P, the inputs cannot form a valid triangle.
5) Can this be used for three-phase systems?
Yes. Choose three phase and enter line-to-line voltage and line current. The calculator uses S = √3 × V × I for apparent power, then derives P and Q using PF or angle.
6) What does the angle φ mean?
φ is the phase angle between voltage and current. Positive angles indicate lagging behavior; negative angles indicate leading behavior. Power factor equals cos(|φ|), so larger |φ| reduces PF.
7) Why is my derived current different from measured current?
Derived current assumes steady-state sinusoidal conditions and uses apparent power S. Harmonics, measurement location, unbalance, or instrument error can cause differences. Confirm whether voltage is line-to-line and whether current is line current.
8) Is reactive power always “bad”?
Not always. Reactive power is required to establish magnetic and electric fields in many devices. However, excessive reactive power increases current, losses, and voltage drop. Correction with capacitors or control can improve efficiency and capacity usage.