In short. A UPS and a generator are compatible only when both are sized and set for each other. The generator must carry the UPS input including battery recharge, the UPS must tolerate generator voltage and frequency, and harmonics and load steps must be controlled.
Roles
UPS protects the instant; the generator protects the duration. The UPS battery bridges the generator start and transfer. The generator then carries the load for the outage.
Sequence on mains failure
T0 mains fails → UPS to battery → T1 start signal → T2 engine runs and stabilises → T3 ATS transfers → T4 UPS rectifier walks in → T5 battery recharges. See ATS/AMF/DG architecture and ATS and AMF logic.
Sizing the generator
Generator kVA ≥ (UPS input kW including recharge ÷ pf) + other loads, with margin for harmonics
UPS input power equals output divided by efficiency, plus charger power. Nonlinear input needs extra margin. See generator sizing and UPS and nonlinear loads.
Compatibility checks
| Item | Why |
|---|---|
| Rectifier walk-in | Avoids a sudden load step on the generator. See transient response |
| Input harmonics | Distort generator voltage |
| Frequency window | Generator drift must stay inside |
| Voltage window | Avoid battery drain on generator |
| Bypass timing | Avoid transfer on poor bypass source. See bypass |
| Battery recharge current | Adds load after transfer |
Commissioning tests
- Simulated mains failure with the UPS loaded.
- ATS transfer and walk-in observed on the generator.
- Return to mains and re-transfer.
- Load bank run with the UPS. See load bank testing.
Also see commissioning.
Illustrative example
A site sizes the generator for the UPS output only. After transfer the generator hunts at the load step, because the charger and input harmonics were ignored. Re-sizing with walk-in enabled and a harmonic filter solves it.
Generator sizing for a UPS load: a worked example
A UPS delivers 40 kW to the load at 94 percent efficiency. The rectifier input power is about 42.6 kW. Add battery charging, say 5 kW at the start of recharge. Total input is about 47.6 kW. Assume a six-pulse rectifier without filter at a power factor around 0.85 on the input.
Generator kVA ≥ Input kW ÷ Input power factor, then add margin for harmonics and other loads
47.6 ÷ 0.85 ≈ 56 kVA from the UPS alone. Applying the commonly used extra margin for nonlinear sources, which depends on the UPS type and any filtering, a generator of 100 kVA or more may be proposed once other loads are added. The correct figure comes from the UPS and generator makers’ guidance. See generator sizing.
Why walk-in matters
Without walk-in, the UPS rectifier loads the generator in one step when the transfer happens. The generator voltage and speed dip, and the UPS may reject the supply as out of window, flipping back to battery. With walk-in, the load rises over seconds. This is commonly an adjustable setting. Check it is enabled.
Frequency and voltage window
| Item | Check |
|---|---|
| Frequency tolerance of the UPS input | Wider than generator drift under load steps |
| Voltage window | Includes generator regulation band |
| Rate of change tolerated | Generator frequency slew is within UPS limits |
| Bypass settings | Allow operation on generator |
Troubleshooting
- UPS stays on battery on generator: check the input window and frequency.
- Generator hunts: check walk-in, governor tuning and harmonics.
- Voltage distortion on generator: consider filters or a larger alternator.
- Repeated transfers: check ATS delay timers and generator stability.
Acceptance test
Run the complete scenario: mains fail, UPS on battery, generator starts, ATS transfers, UPS walks in, battery recharges, mains return, retransfer. Record voltage, frequency and current during each stage. Repeat annually and after any significant change.
The master engineering question
When the generator takes over, what does the UPS ask of it in the first minute?
What to send us
Send UPS data and generator details. Send your application details.
Questions engineers ask
Why does my generator hunt with a UPS?
Load steps, harmonics or governor settings; investigate each.
What is walk-in?
Gradual rectifier loading after the input returns.
Should I oversize the generator?
Size properly with data; do not guess.
Does the UPS reject generator power?
If out of window it stays on battery.
Values are illustrative. Use maker data and site tests.