What is the lowest voltage acceptable ?

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. 🤣

Furman should do just fine. I use older PS Audio regenerator that does voltage regulation too, in addition to other things. They are quite expensive even used and don't always work with powerful power amps, it depends on particular model.

If you're worried about it come and get my TrippLite LC-1800.

I’m using this at present: CyberPower CP1500PFCLCD PFC Sinewave UPS System, 1500VA/1000W, 12 Outlets, AVR, Mini Tower,Black https://a.co/d/08AKYmj9

I’d say 115V should be the lowest acceptable voltage; as others have mentioned, standard US wall outlet voltage is now 120 VAC (half of 240 VAC coming into the breaker panel).

BGE got into a decades-long argument here in Maryland with PEPCO over who was responsible for maintaining/upgrading underground wiring that was going on 40+ years old and, for a long time, we experienced frequent brownouts, blackouts, but only when it got very cold, or very hot, or very wet or, very dry or windy (really). Sometimes we’d lose power for DAYS during ice storms or one particular derecho (wind) storm in JULY about 15 years ago.

I’ve since bought military surplus backup generators. 

We also got power OVERvoltages - as high as 135 VAC - which eventually took out several appliances, a TV and a breaker panel we had to replace; insurance covered, I never found out whether the insurance co was able to subrogate and get reimbursed by BGE for the cost.

On a lark, after the underground wires were finally replaced, I measured power levels day to day. They varied by about 5% around 120 VAC, but mostly stayed at/around 123 VAC with occasional peaks to 125, lows to 118.

These days I use the Cyberpower “conditioner”/UPS to maintain an even 118 VAC as I use some older equipment designed to run on 115 VAC; set any lower and the UPS sets off alarms.

220-221 ... whatever it takes ...