After a show at a club where power in the entire club was wonky, and power on the stage was dipping below 90v during the show, causing my Kemper to reboot THREE TIMES...I decided to research these forums for what folks are using to prevent this. Not much to be found. A few opinions. Mostly from folks who've not actually run into this sort of problem. So I figured I'd just throw up a post for the sake of getting it out there for others who wonder.
I'm using the PowerRack, so we weren't exactly sure how much power I'd need. So we checked out actual draw with a clamp meter. I use a 1x12 cab for stage monitoring the Kemper, and with that thing SCREAMING loud, worst case scenario, I was pulling about 2 amps. Which is quite a bit lower than the plate on the back of the Kemper states as max draw. The math says that 2x120 = 240. I looked around and found a 1U rackmount UPS rated at 300w for right at $140 US. It's made by Cyber Power. Here's a link to their info page.
https://www.cyberpowersystems.com/product/ups/or500lcdrm1u/
I charged it up. Cranked the Kemper and pulled the plug. The readout on the UPS showed that I had about 40 minutes of run time before the battery went dead. It also has an on/off switch so that it's not driving you nuts by beeping every time you unplug it to transport it. The unit is 1U and only 9.25" deep.
The only downside is weight. The UPS adds 18 lbs to the system. But I would much rather deal with that than having this amp rebooting due to poor power conditions.