Why you need both
A UPS and a generator solve different problems. The UPS delivers instant, clean power for minutes — enough to ride through brief outages and bridge the gap while a generator starts. The generator sustains the load for hours or days. For any site that cannot tolerate more than a few minutes of downtime, the two are designed to work as a pair.
The handover
On a mains failure the UPS holds the load on battery instantly. The generator receives an automatic start signal (volt-free contact from the UPS or building management) and typically starts and stabilises within 10–30 seconds. The UPS then synchronises to the generator output and transfers back, recharging its batteries from the generator. When mains returns, the generator runs on briefly to confirm stability before handing back.
Sizing the generator
Size the generator for the UPS load plus charging current, cooling, lighting and future growth — not just the running load. Allow for inrush and the UPS battery recharge draw, which can be significant. As a rule of thumb the generator should be rated at least 1.5–2× the UPS rating to cope with inrush, harmonics and steady-state charging without voltage or frequency excursions that trip the UPS.
Harmonics, fuel and testing
UPS input current can be rich in harmonics, which some generator alternators handle poorly — causing voltage distortion and instability. A low-harmonic UPS (or a 12-pulse rectifier, filter or harmonic-mitigating transformer) keeps the generator happy. Size the fuel tank for the required runtime plus refuelling margin, and test the whole chain regularly: weekly generator no-load runs, annual load-bank tests, and periodic full failover drills prove the system works end-to-end.
Frequently asked questions
Do I need a UPS if I have a generator?
Yes. A generator takes 10–30 seconds to start and stabilise; the UPS holds the load with no break during that gap, then recharges from the generator.
How do I size a generator for a UPS?
Rate it at least 1.5–2× the UPS rating to cover the load, battery charging, inrush and harmonics without voltage or frequency excursions that trip the UPS.
Why does my generator struggle with my UPS?
UPS input harmonics can distort the generator voltage. A low-harmonic UPS, 12-pulse rectifier or harmonic filter resolves this.
How often should I test a UPS and generator together?
Weekly no-load generator runs plus at least annual load-bank tests and periodic full failover drills to prove the whole chain works end-to-end.