I've been having this problem with battery backup units that kick into battery mode repeatedly. They either click or beep every time they do so and it is quite annoying. Looking at the manuals, it seems that there are certain voltage tolerances (ranges) and if the voltage drops or spikes out of that range, it kicks into battery mode. I wanted to see what was happening so I hooked up a digital multimeter to the same receptacle. I could not see the voltage move when the battery backup kicked on (it was still a good 117V at the time). Is it possible that it is happening too fast for the multimeter to pick it up? Or, should I be looking into something else? I've been looking for a good voltage data logger to record a history of voltage but I don't want to bother unless it can pick up these possible quick sags/spikes.