UPS Uninterruptible Power Supply
Battery backup between the wall socket and the equipment. The purpose is rarely to keep working through an outage but to have time to shut down cleanly — and to smooth out spikes and sags.
Three topologies: offline/standby switches to battery only when mains disappears and has a few milliseconds of transfer time, line-interactive also regulates voltage and is the standard choice for servers in smaller environments, online double conversion always feeds from the battery and has no transfer time at all. The VA rating is apparent power — actual watts are lower, often 60–90 percent of it. Runtime is measured in minutes, not hours. Connect it over USB or network to NUT or apcupsd so servers get the signal and shut themselves down. Batteries age and need replacing every three to five years; an untested UPS is false comfort.