The UPS only has to bridge seconds — the gap until the generator starts and takes the load. Size the generator wrong, or the UPS battery too short, and that handoff is exactly where an outage becomes a real one.
UPS runtime and generator sizing solve two different halves of the same outage. The UPS battery's job is to carry the critical load for a short, bridging period — typically just a few minutes — covering the instant of utility failure through the time it takes the standby generator to start, reach rated speed and voltage, and have its automatic transfer switch (ATS) close onto it. The generator's job is everything after that: carrying the full critical load, indefinitely, for as long as fuel is supplied, until utility power returns. Confusing the two leads to two very different, both dangerous, design mistakes: sizing a UPS battery as if it needs to ride out a long outage (wasteful and usually impossible at scale), or sizing a generator without properly accounting for the load characteristics of the equipment it's starting, especially for AI/GPU facilities where load can swing dramatically and quickly.