UPS
A UPS is an Uninterruptible Power Supply. It’s basically a huge rechargeable battery that provides emergency mains power to keep the most important equipment (the servers) running for a short while during a power cut. They typically do not last any longer than 30 minutes unless you’ve bought a really expensive one.
They also generate a really nice smell when the battery cells are burned out. I had to intervene when one Head Teacher was picking up the phone to get the Fire Brigade out.