UPS stands for Uninterruptable Power Supply. This is a machine through which electricity from the mainframe is fed, before reaching the hospital / factory / airport. If the electricity from the mainframe goes down, then the UPS will automatically take over. If it is just a small fluncutation in supply, then the UPS uses stored electricity to cover the need. If it is a power failure of more than a few minutes, then the UPS uses diesel to power the hospital /factory/ airport etc.