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.
Sorry, initailly replied to wrong thread. Not meaning total loss of power, but power fluctuations, brownout, over volt, amps and volts out of phase. You won't normally don't notice because it is short, or not noticable in lights or monitor etc. But with active UPS monitoring you can watch spikes in your power grid. A UPS will see this and rectify power to within normal range. Or an occasional drop in voktage will kick in the UPS to bridge the power drop gap. Then there are major surges that a UPS will buffer by regulation or breaking power to your device.