What is the lowest voltage acceptable ?

in NA the standard voltage is 115v if its a little higher or lower its not a big deal, ie +/-5v. i would not concern your self specially on the slightly lower side. if it goes much lower or higher I'd contact your provider. 

This is governed by your state’s utility regulatory agency, such as a Board of Public Utilities. My state respects the ANSI C84.1 guideline, which calls for a ± 5 percent regulation of the nominal 120VAC service, or 114 volts minimum.

I agree 114 should be the minimum, but the other really hot night Con Edison had some outages a couple of miles away from me and my voltage went down to 109 in the evening. I unplugged my system. The next day it went up to 119 and 115 at night . I emailed my provider ( Con Edison )  and am waiting for a reply.

I use a Furman AR which keeps my line voltage within a consistent 118 to 122 VAC, barring severe issues (in which case it shuts off), as well as protecting me from white outs as well. The most compact/affordable combination is this unit.

In my case, I bought an Elite 15i first, and then realized I needed the voltage regulation, so I have probably spent 3x that much on my setup. Don’t do what I did. 🤣