Apart from bypassing power to the load, Offline UPS also
charges the connected batteries during normal power supply. The bypassing of power results in exposing the load to electrical noises and surges. Therefore, they are not recommended for very sensitive and critical electronic devices. During power failures and other power problems, they switch from grid power to battery power fast enough to prevent computers from rebooting. The switchover time is usually between 2 to 20ms.
As Online UPS always provides power from the battery, the load always gets clean power regardless of any power problems. Therefore, Online UPS is recommended for sensitive and critical electronic devices. Online UPS provide a layer of insulation or electric firewall. Any change in the input voltage or frequency does not affect the output voltage and frequency. Therefore, the load is shielded from all power problems.
source- sa dop