An Offline or Stand-by is the basic idea of an uninterrupted power supply system. Off- Line UPS models protect against power failures only
This kind of UPS switch to battery power in case the voltage is 10% lower than the typical input voltage range. If voltage range comes back to a normal level the UPS switch back to AC power mode.
The benefit of Off- Line UPS is the simplicity and low costs. Installation is easy and quick.
Off- Line UPS are designed for small budgets and less critical loads with a wider input tolerance.