A device used to supply continuous power to critical loads in the event that power from the normal distribution system is lost. The device operates by taking AC from the distribution system and rectifying it to power on a DC bus. An inverter then re-converts the DC to AC that powers the loads. A UPS has a large DC battery supply that is available to be converted to AC power until the normal distribution system is restored or an emergency generator can take over. The more battery capacity a UPS has, the longer it can provide standby power. (see: Inverter; Standby Power Supply; DC Power; AC Power; Rectifier; Incoming Power; Transfer Switch; Distribution System; Generator)