Keeping tradition with doing things backwards, I've finally got a UPS for the rack (mounted in the bottom of the stack). Got a PowerWalker VI 2200R. Its a 2U unit which is all the space I've got left in the rack. Decent price and decent I/O with USB, serial and a slot-in for network expansion + 4 IEC outputs. Its powering everything in the rack and connected via USB to my main server which runs a NUT server that other machines can connect to. A calibration run (100-80%) puts the runtime at about 20 min. Long enough that I'm comfortable setting things to shut down when 20% capacity remains. Summary, I sleep better now.
Now that you have an UPS, put reminders on changing the batteries. My experience with both high end professional units and low cost home units is that batteries last two years more or less and can be expensive to replace.
Having an ups is great, having it fail unexpectedly because the battery packs go from 100% to 1% instantaneously as soon as a load is applied, not so much
I might have been unlucky, but I had mine fail always at the worst possible moment and without the slightest warning.
My current solution is a laptop (but look out for bulging batteries!) And ups backed USB (not cheapo ones!) Jbod or raid enclosures. In this cas e a 50€ battery pack is enough for a brief power outage.
As with all things backups, testing and maintenance is key.