All devices connected to the UPS will suddenly get cut off from power; this is very bad for hard drives!

It was quite panic, as such shut off may cause hard drive damage (actually file structure damage) if hard drive was at same time writing on the disk

I guess I was lucky that the at the time of shut down, my hard drive was not writing on the disk

Running a 2016 iMac, audio interface, Cisco network switch, and iMac peripherals (external hard drives, etc) and still only hitting about 15% of the load, so my battery backup time is calculated to be ~35 minutes.