I was in a Bell Labs facility that had notoriously bad power. We occupied the building before the second main feed had been fully approved by the state and run to the building.

Our main computer lab had a serial UPS that was online 100% of the time, though the inverters where under a very light load. If the mains even acted 'weird' (dips, bad power factor, spikes) the UPS jumped full on, and didn't revert to main power until the main was stable for some duration of time. The UPS was able to carry the full lab (which was quite large) for about two hours, allowing plenty of time for the generator to fire up.

The UPS ran a lot, and because the main was 'weird', the outages were often short, the generator wouldn't even start during the first ten minutes of UPS coverage. Of course, the rest of the building would be dark, other then emergency lighting.

I was a embedded firmware engineer, and our development lab was directly on the wall behind the UPS. When it fired into 100% mode, it roared, mostly from cooling. It was sort of a heads up that the power was likely to fail soon.