I have an APC battery backup for my Star 5, more for the regulated power to the Star 5 and less for the battery back-up. I did one time have a racquet half done when I lost power and was able to finish it with the battery backup with no problem. I had to use a flashlight to get it done and tying off with a flashlight was no fun, but I got it done. My APC will last about an 40 minutes on battery, more then enough time to get a racquet done.
You spend a couple of thousand dollars on the stringer, why not spend $200 to $300 on a APC?