UPS and Generator Handover: What the Battery Must Cover

A generator can be running while the UPS is still using its battery. That gap is easy to miss when a project brief says only that the generator will “start in ten seconds.” Starting is one event. Delivering power the UPS will accept is another.

For a facility that must keep operating through an outage, the battery has to carry the protected load until the UPS can use the generator supply reliably. The useful question is not “How quickly does the engine start?” It is “When does battery discharge actually stop?”

• Count the whole handover, from utility loss until the UPS accepts generator input.
• The ATS and the UPS may apply different tests before accepting the generator source.
• Record the sequence under the intended load; an engine-start figure cannot replace a loaded test.
• After handover, check the generator's response to UPS input and battery charging as well as the protected load.

In a common arrangement, the utility and generator feed an automatic transfer switch (ATS), and the ATS feeds the UPS. When utility power fails, the UPS supports the load from its battery while the generator starts. The ATS waits for the generator source to meet its transfer conditions, then switches. The UPS still has to judge the new input acceptable before its rectifier takes the load back from the battery.

Those events do not happen at the same instant. A site log might look like this:

Illustrative sequence only. In this example, a “ten-second generator start” still leaves the UPS supporting the load for about 40 seconds. The actual sequence depends on the generator, ATS settings, UPS input limits and other loads connected to the generator.

The electrical layout matters too. Some sites use different switchgear or separate UPS rectifier and bypass inputs. Draw the site’s one-line diagram before using any generic timeline.

An ATS generally checks that the alternate source meets its own voltage, frequency and timing settings. The UPS then applies its own input limits. Passing the ATS test does not guarantee that the UPS rectifier will immediately use the generator.

If the generator frequency hunts after a load step, or voltage dips when the ATS connects the site load, the UPS may remain on battery or return to it. An online double-conversion UPS can maintain its output through an unacceptable input, but that does not make the battery reserve unlimited. The input must eventually settle within the range specified for that UPS model.

There is another distinction worth recording during commissioning: the UPS accepting generator power at its rectifier is not necessarily the same as its static bypass being synchronized and available. The rectifier can supply the inverter while the bypass source is not ready for a transfer. Bypass readiness matters if the UPS needs to transfer the load during an overload or fault; check the model's alarms and operating modes rather than treating a running generator as proof that every UPS power path is ready.

This is where an online UPS with generator compatibility needs a project-specific check. Ask for the permitted input voltage and frequency ranges, any generator-mode settings, input current limits and the behavior after source transfer. Do not widen settings simply to stop a battery alarm; first establish whether the generator output and the protected load remain within the equipment makers’ limits.

Measure the interval from utility loss to the point at which the UPS stops sustained battery discharge. Do this with the intended protected load connected. Include the generator start delay, source stabilization, ATS sensing and transfer delays, and any UPS input qualification or rectifier walk-in behavior. Test more than one transfer if the site has variable operating loads.

Use the longest relevant measured sequence as the starting point, then specify the reserve the project needs for cold conditions, battery aging, a slower start or a second attempt. There is no universal extra-minute rule that fits every installation. If the generator fails to start at all, the design also needs a separate answer: how long must the load remain up for an orderly shutdown or another source to take over?

Time is only half the battery calculation. The protected load may change during the handover, and the UPS must carry its kW, kVA and short-duration peaks. Our UPS battery runtime sizing guide shows how to turn a measured load and target minutes into a discharge requirement and check it against battery data. An online UPS with external battery may allow a longer reserve, subject to the approved battery configuration for that model; extra battery does not raise its output rating.

Once the UPS accepts generator input, the generator has to supply more than the protected load alone. The UPS has conversion losses, and its charger may begin replacing energy used during the handover. Other equipment on the same generator can start or cycle at the same time. Those load steps may pull voltage or frequency outside the UPS input window and send it back to battery.

Ask the UPS and generator suppliers how the rectifier ramps its input current, whether charging current can be limited while on generator, and how the generator responds when the UPS and other loads are applied. The answer belongs in the generator sizing study; a simple comparison between generator kVA and UPS kVA is not enough. If outages can repeat, the UPS battery charging guide also explains why the available recharge window matters.

During commissioning, simulate a utility failure under the agreed protected load. Keep a single record of the handover:

Commissioning record

• Utility loss and generator start command
• Generator ready indication, with voltage and frequency readings
• ATS transfer to the generator source
• UPS input acceptance and return to normal rectifier operation
• End of sustained battery discharge

Where battery-current data are available, note when net discharge stops or reverses into charging; otherwise use the UPS event log and mode indication. Save the UPS, generator and ATS logs together. Then check the return to utility, including any retransfer delay or brief return to battery.

If the battery lasts through a no-load demonstration but not a loaded handover, the demonstration did not answer the design question. The useful test is the one that reproduces the operating sequence the site expects to survive.

For an industrial online UPS project, confirm the approved battery configuration before quoting extended runtime. Send enough information to review the whole handover:

Handover quotation checklist

• Site one-line diagram showing utility, generator, ATS and UPS connections
• Measured protected kW/kVA and any load changes or starting peaks
• Required bridge time and the action required if the generator fails to start
• Generator and ATS models, settings and available event logs
• UPS model, input limits and battery configuration

Those details let the supplier size the battery for the real handover, not just the engine’s advertised start time.