This evening, we had a brief power failure. The backup rechargeable flashlight we keep plugged in for times like these came on, and the uninterpretable power supply began making alarm-type noises to tell us "You're on battery backup power! You have 15 minutes to Do Something!"
We began an orderly shutdown of the computers, but by the time I had my computer shut down, the power came back on again. Still, it's nice to know the UPS works in real conditions, as opposed to just testing it.