In an OFFLINE UPS, the load is connected to the mains supply which is stabilized directly. The moment the power fails the load will be switched over to the output of the inverter. There is a definite switch over time in Offline UPS which can be between 5ms to 20ms. Upto 20ms, interruption in power will not be felt while using the computer because the SMPS input capacitor, will supply power to the computer.
In an OFFLINE UPS there is only one transformer which, when the mains is present is used for stabilization of output and also delivers power for charging the battery. When power fails the function is reversed and used as INV transformer.
Microprocessor based technology for high reliability.