The device that maintains continuous psu even on the absence of mains power supply is called PROFESSIONAL UPS or Uninterrupted power supply. This is usually connected between the main source and the working system. It switches from the mains power supply to the battery power instantaneously whenever there is a fault in the mains power supply or the power supply is not there at all. It is generally used to protect typically professional computers, telecommunications equipment, and other electrical equipment where there is a chance of power failure. UPS are available from 200VA onwards to large systems which can support the power supply of multistoried buildings. It is typically the need of all data centers, and computer network systems. Sometimes in large systems it is used in conjunction with industrial generators but it is not to be confused with standby generators. UPS are used for short and instantaneous power supply needs though for longer terms it may be used with a generator.
