A "backup generator" alone is the wrong answer for most premises. The right answer is layered standby — a UPS for the ride-through, a generator for the longer run, and a clear list of which load belongs on which layer.
Three layers
- UPS (uninterruptible power supply) — kicks in within milliseconds; runs 5–30 minutes; carries IT / payment / security systems through a generator start delay
- Standby generator — diesel, typically auto-start on signal from the ATS; runs hours to days while fuelled; carries critical and life-safety loads
- Manual switchover — for less-critical loads that can wait a few minutes for someone to throw a transfer switch
Automatic transfer switch (ATS)
The ATS senses loss of supply, signals the generator to start, waits for the generator to stabilise, and transfers the load. It also reverses the process when normal supply returns. Critical decisions: which loads sit on the standby side of the ATS, and what the ATS does with non-essential loads (drop them, queue them, or maintain on UPS only).
Critical-load schedule
- Life-safety: emergency lighting, fire alarm, fire pump (where electrically driven), smoke control
- Security: CCTV, access control, intercom, alarm panels
- IT / payment: server rack, POS, network switch
- Communications: PABX, public-address system
- Selected lighting and small power — usually a few sockets per floor for staff response
Run-time vs ride-through
UPS ride-through is measured in minutes — long enough to ride out a glitch or a generator start. Generator run-time is measured in hours and is limited by the on-site fuel tank. For longer outages, the contingency plan is fuel resupply scheduling, not a bigger tank. For events and exhibitions, the run-time is the event duration plus a safe margin.
Test on schedule, not on event day
A generator that has not started in six months is a generator that will not start when you need it. Monthly test starts, quarterly full-load tests under the ATS — boring, essential, and the difference between standby that works and standby that exists on paper.
