An Uninterruptible Power Supply (UPS) system is used to provide clean, continuous power when the main source is interrupted or even fails. To ensure the highest-level of power quality and protect the load at all times, the UPS should never need to be switched to raw mains, risking the load.
The level of protection given by the UPS is best quantified by the metric Power Availability. It represents the fraction of time a system is operational during its expected lifetime and we have narrowed down Availability in terms of MTBF/MTBF+MTTR (where MTBF means: Mean Time Between Failure and MTTR means: Mean Time To Repair). In other words, will your UPS work as and when its needed?