In short. Power factor is the ratio of real power to apparent power, PF = kW / kVA. A diesel generator is rated at 0.8 lagging; at lower lagging power factors the alternator overheats before the engine is fully loaded, and at even a slightly leading power factor the voltage regulator can lose control. On generator, capacitor banks should be switched out and the true power factor of UPS and drive loads should be measured, not assumed.
What power factor describes
In an AC system the current drawn by a load is not always in step with the voltage. Motors and transformers draw magnetising current that lags the voltage, carrying energy back and forth without converting it to work. Power factor is the fraction of the total current that does useful work. A resistive heater is at 1.0; a motor at half load may be at 0.7, meaning 30 percent of the current the alternator supplies only magnetises iron.
PF = kW / kVA, and kVAr = square root of (kVA squared minus kW squared)
Here kW is real power, kVA is apparent power, kVAr is reactive power and PF is dimensionless. A 100 kW load drawing 125 kVA has a power factor of 0.8 and a reactive component of 75 kVAr. The relationship is exact for sinusoidal quantities; with distorted waveforms the true power factor also includes the harmonic content, which is covered below.
Why the alternator is rated at 0.8
The alternator’s limit is current and the heat it produces in the windings. The engine’s limit is torque and therefore kW. At 0.8 power factor lagging, a standard generating set reaches both limits together, which is why nameplates read, for example, 250 kVA and 200 kW. Reactive power is supplied by the alternator’s field current, so a low lagging power factor also drives the excitation system harder and raises rotor temperature. Operating at full kVA and 0.6 power factor loads the stator fully and the rotor heavily while the engine idles at 75 percent. The kW to kVA guide shows how the two limits interact.
| Operating condition | Engine loading | Alternator loading | Practical effect |
|---|---|---|---|
| Rated kVA at 0.8 lagging | 100 percent | 100 percent | Design point |
| Rated kW at 0.95 lagging | 100 percent | About 84 percent | Engine limits; alternator runs cool |
| Rated kVA at 0.6 lagging | 75 percent | 100 percent, high field current | Rotor and stator hot, engine lightly loaded |
| Any load at 0.95 leading | Varies | Field current near zero | AVR loses voltage control, voltage rises |
Displacement, distortion and true power factor
Displacement power factor is the cosine of the angle between the fundamental voltage and the fundamental current. It is what a traditional power factor meter reads and what capacitor banks correct. True power factor is real power divided by total apparent power including harmonic currents. Non-linear loads such as variable-speed drives, UPS rectifiers, LED drivers and switch-mode power supplies draw current in pulses rather than sine waves, so their true power factor is lower than their displacement power factor even when the two waveforms are in phase.
For generator work the true power factor matters, because the alternator carries every ampere, harmonic or not. A six-pulse drive may show a displacement power factor of 0.95 and a true power factor of 0.75. Capacitors will not improve the true figure and may resonate with the drive’s harmonics. Measure with an analyser that reports both values and record which one you are using. The harmonics and THD guide and the UPS and non-linear loads guide explain the alternator consequences.
Leading power factor on generator
A leading power factor is the condition a generator tolerates least. When the load is capacitive, armature reaction raises the terminal voltage and the AVR responds by reducing field current. Once field current reaches its floor the voltage climbs out of control. Manufacturers typically allow a leading power factor only in the region of 0.95 to 0.9, and the permissible leading kVAr falls as load falls. The common sources on Indian sites are APFC panels that stay in service when the ATS transfers to generator, long lightly loaded cable runs, and UPS systems with passive input filters that become capacitive at low load.
The remedy is procedural as much as electrical. Interlock the APFC panel so that it is disconnected, or held at its minimum step, whenever the generator supplies the bus. For UPS filters, confirm the input power factor at 25 and 50 percent load with the UPS manufacturer, and if it is leading, disable the filter on generator or size the alternator to accept it. Paralleled sets are more vulnerable than single sets, because a small leading kVAr can push one machine into reverse reactive power and trip it on loss of excitation.
Power factor correction and generators
On the DISCOM supply, correcting power factor to 0.95 or better reduces kVA demand charges, which is why nearly every industrial consumer in Telangana and Andhra Pradesh runs an APFC panel. On generator the economics differ. Fuel consumption follows kW, so correction saves no diesel. It does reduce alternator current, which helps where the alternator is the limiting element, for example a plant at 0.7 power factor on a set adequate in kW but short on kVA. There, fixed interlocked correction sized for the generator condition is better than letting the APFC controller hunt against a source whose voltage moves with every step. Keep the corrected operating point between about 0.85 and 0.95 lagging and never cross unity.
Worked example (illustrative)
A spinning mill records 420 kW and 525 kVA on its generator during a shift, a power factor of 0.8 and 315 kVAr of reactive demand. The 625 kVA set is at 84 percent of alternator rating and the engine at 84 percent of its 500 kW. If the mill adds 60 kW of lightly loaded ring frames that pull the power factor to 0.72, the 480 kW becomes 667 kVA and the alternator is over its rating while the engine is at 96 percent. A fixed 150 kVAr bank, interlocked to connect only on generator, would bring apparent power back to about 540 kVA at 0.89 lagging and restore alternator margin without touching the fuel bill. Confirm the figures against a measured load profile before sizing the bank.
Common mistakes
- Leaving the APFC panel in automatic mode on generator, which steps capacitors in at light load and drives the set into a leading power factor and voltage runaway.
- Reading displacement power factor from the panel meter and using it for drive-heavy or UPS loads, so the alternator carries 20 percent more current than the sizing assumed.
- Adding capacitors to a generator-fed bus to save fuel, which saves nothing because fuel follows kW and introduces resonance risk with harmonic loads.
- Specifying a set on kW alone for a plant with lightly loaded motors, so the alternator overheats at 0.7 power factor while the engine shows only 70 percent load.
What to send us
Send a power analyser log or the DISCOM bill showing kW, kVA and power factor, details of any APFC panel and how it is controlled, and the make and rating of UPS systems and large variable-speed drives on the generator bus. A photograph of the generator nameplate and of the controller display while the set is on load lets us check the actual operating point against the alternator’s capability. Send your application engineering enquiry.
Questions engineers ask
Why is 0.8 the standard generator power factor?
It represents a typical industrial load mix and is the point at which the engine’s kW limit and the alternator’s kVA limit are reached together. Loads with a higher power factor are limited by the engine; loads with a lower power factor are limited by the alternator.
Can a generator run at unity power factor?
Yes, a purely resistive load at unity power factor is fine and loads the engine to its full kW with the alternator running cool. The problem begins only when the load goes leading, which is capacitive, not when it reaches unity.
Should the APFC panel stay on when the generator is running?
Generally no. Interlock it so that it is off, or held at a fixed minimum step that keeps the power factor lagging, whenever the generator is the source. Automatic switching against a generator causes voltage instability and nuisance trips.
Does improving power factor reduce generator fuel consumption?
Not noticeably. Fuel consumption is set by real power in kW. Correction reduces alternator current and heating, which matters when the alternator is the limiting element, but the engine burns the same diesel for the same kW.
How do I measure true power factor?
Use a power quality analyser that reports both displacement and true power factor, logged over a representative shift. A conventional panel meter usually shows displacement only, which overstates the real figure where drives, UPS and LED loads are present.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.