Offline UPS and Online UPS Basically there are two types of UPS. These are Offline UPS and Online UPS. Off line UPS passes the input AC to the output sockets if the AC power is available. It always monitor the voltage level in the mains, and if there is a voltage drop or mains failure, it switches on the inverter to give AC power to the device until the mains supply returns to normal. The switch over time from AC to inverter AC is less than five milli seconds so that the functioning of the gadget is not affected.