Line interactive UPS systems monitor the incoming power, if it detects a fluctuation or a failure, it either boosts or decreases power using automatic voltage regulator or runs on batteries. Having the system bypassing the utility directly to load, during power outages or failures, there would be a short interruption of up to 4 milliseconds, which is fast enough to keep most of the equipment operating without interruption.
Online UPS also known as double conversion, provides the highest level of protection by isolating load from utility. The system works by continuously converting AC to DC and then inverts it back to AC. Having the system continuously operation, there is a zero interruption period between power outage and operation on batteries.