In short. A generator feeding a UPS sees the rectifier input, not the UPS output nameplate. The real demand is the supported load divided by UPS efficiency, plus battery recharge, drawn with whatever harmonic distortion the rectifier produces. Because an alternator has far higher source impedance than the grid, that distortion becomes voltage distortion, which upsets AVRs and the UPS input window. Size from the rectifier type, charger setting and alternator data, not from a fixed multiplier.
What the generator actually sees
A UPS is two converters in series with a battery between them. The load sees the clean inverter output; the generator sees the rectifier input, a different load altogether. Three things set its size. The first is the real power delivered to the critical load, divided by the UPS efficiency, typically 0.93 to 0.96 for modern double-conversion units at normal loading. The second is battery recharge, which begins the moment the generator takes over because the battery has just been discharged during the start and transfer gap. The third is the shape of the current, because a rectifier does not draw a sine wave.
The same thinking applies to every non-linear load on the site: six-pulse variable-frequency drives, switched-mode supplies in servers and LED drivers, telecom rectifiers and EV chargers. Each draws current in pulses, the pulses add up at the generator terminals, and single-phase electronic loads also push third-harmonic current into the neutral, which is why full-size neutrals and a two-thirds pitch alternator are specified for such sites.
Estimating generator demand from a UPS
Pgen = (Pload ÷ ηUPS) + Precharge
Here Pgen is the real power the generator must supply in kW, Pload is the actual critical load carried by the UPS in kW, ηUPS is the UPS efficiency as a fraction, and Precharge is the battery charging power in kW, which the UPS data sheet states as a percentage of rating or as a maximum charger current.
Worked example (illustrative). A 100 kVA UPS rated at 0.9 power factor carries 72 kW of server load. At an efficiency of 0.94 the rectifier input is 76.6 kW. The charger is set to 10 percent of the UPS rating, adding about 9 kW, so the generator supplies roughly 86 kW for the UPS alone, or about 87 kVA at an input power factor of 0.99. Add the cooling, lighting and other essential loads that transfer at the same time, then compare the total with the generator rating. The UPS nameplate of 100 kVA was never the number that mattered.
Rectifier technology decides the harmonic burden
Older thyristor units with six-pulse bridges draw heavily distorted current, and a generator serving them needs a large margin. Twelve-pulse bridges and passive input filters reduce the distortion. Modern IGBT or transformerless rectifiers draw nearly sinusoidal current at close to unity power factor, and the margin shrinks to little more than the recharge allowance. The table gives screening ranges that are widely quoted in the industry. They start a discussion; the UPS manufacturer’s generator compatibility note and the alternator manufacturer’s derating for distorted loads finish it.
| Rectifier type | Typical input current THD | Typical input power factor | Screening ratio, generator kVA to UPS kVA |
|---|---|---|---|
| Six-pulse thyristor, no filter | 25 to 35 percent | 0.80 to 0.85 | 2.5 to 3.0 |
| Six-pulse with passive filter | 8 to 12 percent | 0.90 to 0.95 | 1.6 to 2.0 |
| Twelve-pulse | 10 to 12 percent | 0.85 to 0.90 | 1.5 to 1.8 |
| Twelve-pulse with filter | About 5 percent | 0.95 or better | 1.3 to 1.5 |
| IGBT or transformerless rectifier | 3 to 5 percent | 0.99 | 1.1 to 1.4 |
The ratios exceed the simple power arithmetic because the alternator, not the engine, is usually the limiting component: harmonic currents heat the stator and damper windings and distort the terminal voltage. Where the alternator is the constraint, the next frame size of alternator on the same engine is often cheaper than a larger generating set. See alternators and non-linear loads for winding and pitch considerations.
Why the grid tolerates what a generator does not
A DISCOM supply through a distribution transformer has a source impedance of a few percent on the transformer base. A generator’s subtransient reactance is typically 10 to 20 percent on its own base, and its transient and synchronous reactances are higher still. The same distorted current therefore produces several times more voltage distortion at generator terminals than at a utility bus. Analogue AVRs misread the distorted waveform and regulate to the wrong value, the rectifier sees a voltage outside its window and transfers to battery, the load leaves the generator, the voltage recovers, the rectifier returns, and the cycle repeats. The mechanism is explained in harmonics and THD.
Input window, frequency slew and the bypass path
Every UPS has an input acceptance window, commonly around plus or minus 10 to 15 percent on voltage and plus or minus 5 percent on frequency, with a tighter window before the static bypass is considered available. Most units also limit the rate of frequency change they will follow, often around 1 Hz per second in generator mode. An engine that hunts under a step load pushes the supply outside the window, the UPS runs on battery while the generator runs, and the batteries never recharge. Commission the UPS with a generator-mode profile that widens the window, reduces charger current and inhibits bypass while on generator.
Load steps during recovery
The sequence after a mains failure is a series of steps: the AMF panel transfers the essential bus, the rectifier walks in over 10 to 20 seconds, the charger resumes at full rate, then cooling plant restarts with its motors. Each step draws a frequency and voltage dip, described in generator transient response. Staggering the steps in the control logic costs nothing and avoids most nuisance transfers.
Leading power factor at light load
Transformerless UPS units and filtered rectifiers can present a leading power factor when lightly loaded, because their input filters stay connected while the rectifier draws little current. Alternators absorb only limited leading reactive power before the voltage rises, often around 0.95 leading at reduced load. A large UPS carrying a small load during testing is the classic case, and it appears as an AVR overvoltage trip.
Common mistakes
- Sizing the generator from the UPS output rating alone; the generator then runs overloaded during recharge and the UPS cycles to battery.
- Leaving the battery charger at its default rate on generator supply; the recharge step dips the voltage and trips the rectifier off.
- Testing with the UPS at light load and declaring compatibility; the real problem appears at full load on a hot afternoon.
- Ignoring the alternator’s harmonic derating because the engine kW looks sufficient; the alternator overheats and the AVR becomes unstable.
What to send us
Send the UPS make, model and rating, its rectifier type, the measured or expected critical load in kW, the charger setting and battery capacity, and the make and frame of the existing or proposed alternator with its reactance data if available. Include a single-line diagram showing which other loads transfer to the generator at the same time. If an existing installation cycles to battery, add a photograph of the UPS event log and the generator controller alarms. Send your application details and we will review the combination before anything is purchased. The generator sizing guide covers the rest of the load.
Questions engineers ask
Why does my UPS switch to battery when the generator is running?
The generator supply is falling outside the UPS input window, usually because of voltage distortion, a frequency excursion on a load step, or a slew rate faster than the UPS will track. Read the reason code in the UPS event log, then look at alternator loading, AVR type and the charger step. A generator-mode profile on the UPS and a staged recovery sequence usually resolve it.
How much larger than the UPS should the generator be?
It depends on the rectifier. An IGBT rectifier may need a generator only 1.1 to 1.4 times the UPS rating, while an old six-pulse unit may need 2.5 to 3 times. Then add every other load that transfers with it, and use the manufacturer’s compatibility statement rather than a universal multiplier.
Can battery recharge be limited while on generator?
Most modern UPS units accept a dry contact from the AMF panel that signals generator operation and switches the charger to a reduced current, slows the rectifier walk-in and widens the input window. Wire and test it at commissioning, because the default charger rate assumes a stiff utility supply.
Is a two-thirds pitch alternator necessary?
For loads with significant single-phase electronic content it is strongly preferred, because the two-thirds pitch winding suppresses the third harmonic that would otherwise circulate in the neutral and distort the line-to-neutral voltage. Confirm it on the alternator data sheet rather than assuming.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.