Backup power supply that, in case of power failure or fluctuations, allows enough time for an orderly shutdown of the system or for standby generator to start up. UPS consists usually of a bank of batteries and power sensing and conditioning circuitry, and comes in two basic types: (1) Continuously-Online UPS provides steady and clean (spike free) AC power even when the mains power supply is available, and is recharged continuously by the mains supply. Also called true online backup. (2) Standby UPS automatically comes on when the mains supply fails or fluctuates inordinately.