UPS and generator coordination: what happens when the mains fails?
A UPS and standby generator each work perfectly in isolation, but how do they work in unison when the mains fails? We examine the critical handover between UPS and generator power.
For organisations requiring considerable runtimes from their power protection equipment, resilience often relies on two systems working together: an uninterruptible power supply (UPS) and a standby generator. The principle is straightforward. The mains fails, the UPS immediately supports the critical load from its batteries, the generator starts and, once its output is stable, takes over the electrical supply. In practice, those few seconds involve a carefully coordinated sequence – simply having both a UPS and generator doesn't necessarily guarantee that they will operate effectively together.
Bridging the gap between mains and generator
A standby generator cannot provide instantaneous backup power. Its engine needs time to start, reach operating speed and establish a stable voltage and frequency before it can accept the load. The UPS bridges this gap. When the mains supply fails, the UPS continues supplying the connected critical equipment from its batteries. Once the generator is available and the site's transfer equipment has operated, the UPS accepts the generator supply and returns to normal operation. This is why UPS battery autonomy remains important even where a standby generator is installed. The batteries need sufficient capacity to support the critical load throughout the generator start and transfer sequence, with appropriate contingency should the generator be delayed or fail to start.
Correct sizing matters
UPS and generator requirements should be considered as part of the same overall critical-power strategy, although the two systems may support very different loads. The UPS will typically protect equipment requiring uninterrupted power, such as servers, IT infrastructure, CCTV or other critical systems. The standby generator, however, may be required to support a much broader electrical load across the site. Generator sizing should therefore be based on the total load it is expected to support during a mains failure, rather than simply being matched to the maximum kVA capacity of the UPS.
Testing the complete system
Routine UPS servicing and generator maintenance are essential, but testing the two systems independently doesn't necessarily prove that the complete backup-power arrangement will work. For critical sites, controlled integrated testing, where operationally viable, can provide additional assurance by confirming the full sequence: loss of mains, UPS battery operation, generator start, automatic transfer, operation on generator power and eventual return to the utility supply. The objective is simple – to demonstrate that the entire backup-power chain works as intended, rather than confirming that its individual components work in isolation.
Maintaining your critical-power resilience
Backup-power requirements also change over time. Electrical loads increase or decrease, equipment is replaced and systems originally designed years ago may no longer reflect the needs of the organisation. Regular preventative maintenance, combined with periodic review of the overall resilience strategy, can identify developing weaknesses before they result in disruption. The important question isn't simply “Does our UPS work?” or “Will our generator start?" It's, “What happens to our critical operations from the moment the mains fails until normal power is restored?”
NSSE provides UPS and standby generator solutions covering system design, supply and installation, preventative maintenance, battery replacement and emergency support. If your UPS and generator systems have traditionally been assessed or maintained separately, NSSE can review your complete backup-power arrangement and help identify potential weaknesses before they are discovered during a genuine power failure.
