UPS or Uninterruptible Power Supply is a device which provides immediate backup power during a power failure.

For larger scale developments such as hotels, residential developments and offices this is necessary to ensure there is no risk to health or disruption to services wich as data centres, emergency lighting, fire alarms and other safety systems, 

A UPS is different to an auxiliary or emergency power supply in that is provides almost instantaneous supply avoiding any interruption. 

The system can be of significant size & weight and this needs to be considered spatially early in the design, but typically requires less plant space in addition to lower capital cost over generator installation.