This was originally posted on APC forums on 4/7/2017
108 volts is low, but it is within tolerance (+/- 10%) for equipment designed for use on a nominally 120 volt system.
The AVR capability on this model should kick in somewhere around 103 volts AC. I find it interesting that the manual indicates "boost only", as I thought the larger (1300/1500 VA) Back-UPS models had both boost and buck (voltage reduction) capability. Perhaps that has been dropped in newer production?
AVR works by employing what is known as an autotransformer. An autotransformer has multiple taps, or voltage outputs, that may be utilized to maintain the output voltage going to a given device at a certain level. The UPS's microcontroller would "decide" which tap would be placed into circuit to maintain the line voltage at the desired level. The transformer is rated to perform this operation continuously, so the UPS won't have to switch to battery, and you won't have to worry about shutting your equipment down before the battery would normally run out.
AVR will turn on automatically when the line voltage drops too low (or in models so equipped, rises too high) and it will turn off automatically when normal line voltage levels are restored.
If AVR cannot compensate for a low or high voltage condition (it usually only has one or two steps worth of adjustment) the UPS will switch over to battery. At this time the on-battery alarm would sound, indicating that you will eventually have to turn your equipment off.
I'm not sure if APC's PowerChute software alerts you when AVR is being used. Normally, the only sign that AVR might be operating would come in the form of some relay clicks emitted by the UPS, and the AVR indicator being illuminated on the unit's status display.
You can adjust the unit's sensitivity from the front panel controls or with PowerChute software if you are using it. This will also adjust the points where AVR kicks in.
