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.