[bombcar]

So the one I was in had these massive rooms full of lead-acid batteries, and 3 or 4 multi-megawatt "generators" outside (located at various sides of the building, not all in one place).

They announced it over the PA because it was a planned transfer - and then all the "normal" lighting went off (think overhead fluorescents) at the same time the emergency lighting came on - the power to the servers and anything plugged into the racks didn't even flicker, of course.

There was a huge thunking sound from somewhere (I assume something disconnecting from grid power) and the muted roar of the generators starting. A few moments later, the main lights came back on.

It was an entirely different experience than a normal power failure, where everything goes quiet - you knew something happened, but other than that it all worked smoothly.