This unit performs the same operation as household inverters. Inverter circuit converts switched DC power from a rectifier or backup batteries to again an AC output that powers the sensitive electronic gadgets.
The whole conversion process (from AC to DC and again DC to AC) and filtering smooth out events such as spikes, surges, and electrical noise impulses that is ensuring the final smooth, controlled, and regulated pure sine wave (AC power) to sensitive loads. In this way, Online UPS protects electronic gadgets.
Despite the all safety features, there is one possibility of failure is present. And that is the failure of UPS or uninterrupted power supply. In that scenario a static switch automatically connects the load to the mains supply, bypassing the rectifier, batteries, and inverter circuit.