Power factor is the ratio of real power (kW) to apparent power (kVA), equal to the cosine of the phase angle between voltage and current. It measures how much of the current you draw actually does useful work.
The useful power that actually does work — turning motors, heating elements, and lighting lamps. It is what your appliances consume and what you pay for on your utility bill.
The total power a generator must supply. It is the vector sum of real power (kW) and reactive power (kVAR). Generators are rated in kVA because their alternators are sized by current.
The non-working power that circulates between the source and the load. It does no useful work, but it still flows through cables, switchgear, and generator windings — requiring a larger generator for the same kW.
Think of it as a beer glass. The kW is the beer (the useful part). The kVAR is the foam (the unusable part). The kVA is the whole glass — you have to pay for a big enough glass to hold both. A power factor of 0.8 means your glass is 20% foam.
The ratio of real power to apparent power. A PF of 0.8 means 80% of the current is doing useful work — 20% is reactive, contributing nothing but heat and requiring a larger generator.
This is the conversion that matters for generator sizing. A 100 kW load at 0.8 PF requires 125 kVA. At 0.7 PF, the same load requires 143 kVA — a larger generator for the same work.
Reactive power is the third ngle. It flows back and forth in cables and windings, creside of the power triaating heat and voltage drop without doing any useful work. Reducing kVAR is the goal of power factor correction.
Resistive loads (heaters, lighting) have PF ≈ 1.0. Motors, VFDs, and transformers typically run between 0.7 and 0.85. A poor power factor means more current for the same kW — and a larger generator.
The calculator runs on three interchangeable formulas, each the same relationship from a different angle. Enter any two values, and the tool returns the third. In three-phase systems the same relationship holds — the difference is only in how you reach the current.
Enter kW and power factor to get kVA. Or enter any two of the three values — kW, kVA, or PF — and the tool returns the missing one.
Enter your current kW, power factor, and target power factor. The tool returns the kVAR of capacitors needed and the kVA saved on your generator.
The table below shows how power factor changes the generator size for the same real load. Run your own numbers in the calculator, but this is the shape of it.
| Real Load (kW) | Generator to Specify | Power Factor | Apparent Power |
|---|---|---|---|
| 80 kW | 100 kVA | 0.80 | 100 kVA |
| 100 kW | 125 kVA | 0.80 | 125 kVA |
| 250 kW | 300 kVA prime / 330 kVA standby | 0.85 | 294 kVA |
| 100 kW | 125 kVA | 0.90 | 111 kVA |
| 400 kW | 600 kVA | 0.70 | 571 kVA |
| 500 kW | 625 kVA | 0.80 | 625 kVA |
Generators are rated at 0.8 power factor lagging because that matches the inductive loads they typically serve — motors, transformers, and fluorescent lighting — and because 0.8 is the heat limit at which the alternator and engine can run together continuously. A 100 kVA alternator at 0.8 PF is matched to an 80 kW engine.
The 0.8 figure is a rating convention, not the actual load. It was chosen because most industrial loads are inductive and typically operate in the 0.75–0.85 PF range. The 0.8 rating balances engine kW and alternator kVA for continuous operation.
Below 0.8 lagging, the alternator needs more rotor excitation to hold the voltage. The rotor — one of the hardest parts to cool — runs hotter, forcing a derate. At 0.8 PF, the alternator and engine reach their thermal limits together.
A generator can run between about 0.8 and 1.0 power factor without derating. Above 0.8 is safe; below 0.8 requires a larger alternator for the same kW, or a generator size increase to maintain the same kVA.
A 100 kVA generator at 0.8 PF delivers 80 kW of real power. This is the relationship on every nameplate: kW = kVA × 0.8. If your load has a power factor below 0.8, the nameplate kW is not the kW you will get.
A 400 kW motor-heavy load at 0.7 PF needs 571 kVA, not 400 kVA — a far bigger generator than the kilowatts alone suggest. The calculator makes this conversion visible instead of letting it hide inside a nameplate.
A 400 kW load at 0.7 PF requires 571 kVA. At 0.9 PF, the same load would require just 444 kVA — a full generator size smaller. The difference is not the load; the difference is the power factor.
Most of the time the danger is a low power factor. But there is a second, less obvious failure — leading power factor — and it is worse, because it usually arrives as an accident. A generator can tolerate only slightly leading conditions — around 0.95 to 1.0.
A leading power factor causes the magnetic field at the ends of the stator windings to overheat. This is a thermal failure mode that is not always protected by the generator's standard overcurrent protection.
The voltage regulator loses its reference and hunts. This can cause voltage fluctuations, flicker, and in some cases, complete loss of voltage regulation — particularly on lightly loaded sets.
The rotor slips a pole, with a risk of generator overspeed and shutdown. This is a genuine emergency that can trip the set offline when you need it most.
Leading power factor happens when the load is capacitive, which is exactly what a power factor correction capacitor bank adds. On the grid, that is harmless. On a generator, it can be destructive.
A lightly loaded generator with a large fixed capacitor bank can easily be driven leading. The controller may trip on overspeed. This is why capacitor banks on generator-fed systems must be switched with the load, not left permanently connected.
The calculator flags leading power factor with a warning so you catch it before the site does. A leading PF is not just inefficient — it is a protection trip waiting to happen.
If your load runs at a poor power factor, you do not always need a bigger generator — you can correct the power factor and free the kVA you already have. The kVAR of correction required is: Qc = P × (tan φ₁ − tan φ₂).
Qc = P × (tan φ₁ − tan φ₂)
Where P is the real power in kW, φ₁ is the angle of the current power factor, and φ₂ is the angle of the target power factor (usually 0.95 to 0.98). The result is the kVAR of capacitors needed.
φ₁ = arccos(0.72) = 43.95°, tan φ₁ = 0.964
φ₂ = arccos(0.95) = 18.19°, tan φ₂ = 0.329
Qc = 500 × (0.964 − 0.329) = 318 kVAR
This frees 170 kVA — from 694 kVA to 526 kVA.
φ₁ = arccos(0.78) = 38.74°, tan φ₁ = 0.802
φ₂ = arccos(0.95) = 18.19°, tan φ₂ = 0.329
Qc = 250 × (0.802 − 0.329) = 118 kVAR
A 118 kVAR capacitor bank lifts the load from 0.78 to 0.95.
Do not over‑correct past 1.0 — that is exactly the leading-power-factor danger from the previous section. Over-correcting can cause stator end-iron heating, AVR instability, and pole-slip trips.
If the site has non‑linear loads (VFDs, UPS), capacitor banks can resonate. A detuned reactor (7% or 14%) may be needed. Our engineers specify the bank and its switching with your generator, so the correction helps rather than destabilizes it.
Final power factor selection should be checked against the load's actual power factor, the generator's rating, and the risk of leading power factor. Use the power factor calculator to convert kW to kVA, find any missing value, and calculate the kVAR of capacitors needed. For capacitor banks on generator-fed systems, always switch with the load and avoid over-correcting past 1.0.
Power factor is not a footnote on a spec sheet; it is the number that turns your kilowatts into the kVA you actually have to buy. Understand it, and you buy the right generator. Ignore it, and you either overpay for capacity you never use or underbuy and watch the machine trip.
A: Power factor is the ratio of real power (kW) to apparent power (kVA), equal to the cosine of the phase angle between voltage and current. It measures how much of the current drawn does useful work, with 0.8 meaning 80% real and 20% reactive.
A: Diesel generators are typically rated at 0.8 power factor lagging. A 100 kVA generator at 0.8 PF delivers 80 kW of real power. The generator can run between about 0.8 and 1.0 PF without derating.
A: Because 0.8 lagging matches the inductive loads generators usually serve, and it is the heat limit at which the alternator and engine run continuously. Below 0.8 lagging, extra rotor excitation overheats the alternator and forces a derate.
A: Divide the kilowatts by the power factor: kVA = kW ÷ PF. For example, 250 kW at 0.85 PF is 250 ÷ 0.85 = 294 kVA. For three-phase systems, the same relationship holds before you convert kVA to current.
A: Leading power factor occurs when the load is capacitive, drawing current that leads the voltage. It is the opposite of the normal inductive (lagging) case and is usually introduced by power factor correction capacitor banks.
A: A leading power factor can cause stator end-iron heating, automatic voltage regulator instability, pole-slip, and generator overspeed or shutdown. Generators tolerate only slightly leading conditions, around 0.95 to 1.0.
A: Add a capacitor bank sized with Qc = P × (tan φ₁ − tan φ₂), where φ₁ and φ₂ are the angles of the current and target power factors. A 500 kW load at 0.72 corrected to 0.95 needs about 318 kVAR.
A: Multiply your real power in kW by the difference in the tangent of the phase angles, tan(arccos(current PF)) minus tan(arccos(target PF)). A 250 kW load at 0.78 corrected to 0.95 needs roughly 118 kVAR.
