Personal machine is MP5,1 with dual 3.46, 128GB RAM, 7 SSDs (1 NVMe), RX580, USB. At idle after startup, machine is around 280-325 watts.
A 1500VA/900W UPS battery backup will allow AT MOST 20 minutes run time, but that is the mathematical calculation (and via onboard display). Real world gives 5-7 minutes maximum before you need to shutdown.
Well this depends on your goals....
Are you wanting to be able to continue to work through a power flicker or an actual extended outage?
If you need more than a couple minutes to shutdown then maybe look for some kind of generator or extended battery system, or sleep the machine and you should drop power usage significantly and typically faster than a shutdown and quicker startup.