Line interactive uninterruptible power supply (UPS) systems pass through the input power until there is a power event that triggers either backup power or automatic voltage regulation to kick in. Depending on the manufacturer, this takes anywhere from 4 milliseconds to 12 milliseconds to happen. Newer sensitive and energy efficient electronics like those with Active PFC or an Energy Star 5.0 or higher rating sometimes lose power anyway if connected to line interactive systems with higher than an 8 millisecond delay. You won’t find out though until the UPS is tested with the device or the first power event occurs as power is passed through on line interactive units. Battery Backup Power, Inc. line interactive systems switch over in 4 milliseconds or less which is why they are typically more expensive and heavier than lower grade consumer uninterruptible power supplies.
Double conversion (also referred to as online) uninterruptible power supplies have absolutely no delay in switching to backup power or filtering out electrical problems because they are always online. They typically continuously convert the input AC power to DC power (for the batteries), then take the DC power from the batteries and convert it back to AC power for the attached electronics. This separates power issues at the input source (grid power, generator, etc.) from the output of the UPS providing clean, continuous power no matter what happens on the input side. There is also additional electrical noise filtration, voltage regulation, frequency correction, and other power conditioning that occurs depending on the unit purchased. These units are used to backup and filter out power problems for critical electronics and ultra sensitive electronics. Their output power is equivalent or better than what comes out of a wall socket, so any electrical device will work with this type of UPS. This is why NASA uses a Battery Backup Power, Inc. double conversion unit at its Moon Rock Analysis lab in Huntsville, Alabama (see below picture).