Once upon a time those of us who mess with computers called these things "uninterruptible power supplies". It was--- and is-- a battery backup system designed to give you a bit of power in case the main power goes out. It uses deep-cycle batteries powering an inverter that gives you AC power. These seem to be a bit bigger than the ones I saw last, but the idea is the same. The ones I used to be familiar with would give you at least enough time to save and shut down any open applications before shutting off the computer safely, and a few would allow a couple of hours worth of work provided you didn't insist on trying to power every electrical device in the office.