The purpose of a UPS is to smooth the voltage fluctuations and prevent major outages from minor power interruptions that would otherwise potentially harm your switches.
The standard UPS's we provide are the APC2200 - these can be either rack mounted or as a tower.
Depending on the power consumption of the switches ie, number of slots you have, along with the number of phones and WAPs that are patched in - the life of the battery will vary the amount of uptime that can be provided.
The small UPS provided by essensys would not be expected to deliver more than 5-10mins of uptime in the event of a power failure, and depending on the switch load this will also change and fluctuate with time.
In other words, the more load you place on the UPS, the shorter it's battery will last in a power failure.
It's important to note that they cannot deliver power on an ongoing basis - e.g. 20 mins or more.
It is also recommended that the UPS battery be replaced every 3 - 5 years.