An Uninterruptible Power Supply (UPS) is a device that sits between a power supply (e.g. a wall outlet) and a device (e.g. a computer) to prevent undesired features of the power source (outages, sags, surges, bad harmonics, etc.) from the supply from adversely affecting the performance of the device.

There are mainly three topologies of UPS i.e. Offline, Line-interactive & offline UPS.

Online UPS involves the use of major part of the online power through an inverter. Inverter remains always "On" during normal operation of UPS. Due to which there will be zero transfer time from mains to battery and vice versa. This makes it true uninterrupted power supply.

This is type of online UPS in which mains AC power subject to first convert from AC to DC power with help of Rectifier and then DC to AC power with help of Inverter.

A UPS has internal batteries to guarantee that continuous power is provided to the equipment even if the power source stops providing power. Of course the UPS can only provide power for a desired time (depending on battery backup time). Even if the outage is longer than the battery lifetime of the UPS, this provides the opportunity to execute an orderly shutdown of the equipment. Advantages:

A UPS traditionally can perform the following functions:

It is mainly depends on for how much time battery system is designed for. It also depends on percentage of load.