UPS Runtime and Generator Handover: Plan for the Whole Gap

1. Identify the equipment that must stay on and the power it actually draws.

2. Time the full generator start and transfer sequence, not just the engine start.

3. Check battery runtime at that load; a UPS kVA rating does not specify minutes.

A generator may be ready to start when the grid fails. That does not mean the equipment it protects will see a continuous supply. The UPS has to carry the load until the generator is running, its output is acceptable and the supply has transferred.

At its October 2026 Electricity Security Advisory Board meeting, the International Energy Agency brought together experts to discuss grid infrastructure, storage, adequate supply and affordability. The IEA was discussing power systems, not prescribing UPS designs. For a business or facility, however, electricity security begins with a narrower question: what keeps essential equipment running when its normal supply is interrupted?

“Keep the server room running” is a useful goal, but it is not yet a UPS specification. Which servers and network devices must stay on? Are there control systems elsewhere in the building that share the same protected supply? What power do these loads draw in normal operation?

The answers establish the load the UPS must carry. Its kVA rating sets an output limit; it does not tell you how many minutes the batteries will last. Battery runtime changes with the connected load and the battery configuration, so both need to appear in the selection.

The required time comes from what happens after the outage. Staff may need only enough time for an orderly shutdown. A site that must keep operating until another source takes over needs to cover the full handover, with a margin appropriate to that site’s start and transfer arrangements.

The battery supports the load while the generator starts, reaches usable voltage and frequency, and the supply transfers. Planning around the advertised engine start time alone can leave part of that interval uncovered. The site should also decide what happens if the first start attempt fails.

An external battery design lets the UPS output capacity and the battery arrangement be selected for different requirements. TAFENG’s BLAZING POWER line includes a 6 kVA online UPS with an external battery design. Its required battery configuration still has to be calculated from the actual load and target runtime.

The test does not end when the generator starts. Once it supplies the UPS, the generator’s output must remain within the UPS input limits under the site’s operating conditions. That is something to verify during commissioning, with the protected load connected.

Some sites have no generator. Others need their equipment to run well beyond the handover period. In those cases, the battery bank becomes a larger part of the installation decision. Its space, weight, recharge time and replacement access should be considered before a runtime is promised.

The same distinction applies to larger three-phase systems. A 40 kVA low-frequency online UPS serves a different power range from a 6 kVA unit, but its battery runtime must still be specified separately. A larger UPS rating does not, by itself, mean longer backup.

For a useful quotation, give the supplier the equipment list, its measured or reliably estimated load, and the minutes it must continue operating after an outage. If a generator is involved, include the start and transfer sequence. Those details turn “backup power” into a requirement that can be calculated and tested.