In this type of UPSs the load is directly connected to the incoming AC power supply. When this mains supply fails or goes below a minimum level, the offline UPS blocks the incoming AC mains and deliver power to the load via internally connected battery using DC-AC inverter circuitry. Although all manufacturer tries to minimise the switching delay but it could be as long as 25 milliseconds. Generally for home desktops we use offline UPS.
This type of UPSs always deliver power to the load via battery using DC-AC inverter. Therefore in these UPSs no switching mechanism required, and hence transfer time has no role during power failure. To maintain the charge of the battery, a battery charging unit is incorporated in the system. So when mains supply fails, UPS continues to deliver power to the load using battery, however, charging of battery stops. This is what happens when a plugged-in laptop keeps on running without interruption when mains power fails.
In the power range between 750VA and 5000VA, both types of UPS adequately protect IT equipment from power disturbances, so the decision for which topology to use is primarily based on the specifics of the customer application.
Here is a comparison chart of Online and Offline UPS mentioning advantages (+) and disadvantages (-)
lower operating temperature
Lower initial cost (fewer parts)
Lower operating cost (less electricity)
Extreme voltage distortion can require frequent battery usage
Output frequency varies within a configurable range
higher operating temperature
Higher initial cost (more parts)
Higher operating cost (more electricity)
Accepts extreme voltage distortion without going to battery
Output fixed to a configurable frequency