In your case you have already exhausted the UPS battery to a very low level so it may or may not provide any support during the period of instability as line power is restored. Therefore your immediate boot could be interrupted by a subsequent loss of power and I am ignoring the probability of huge spikes that might get past the filters in the UPS.
You'd have to see my setup to understand. There's a very large isolation transformer between the UPS and the power bar. (basketball size, weighs about 70 lbs) It means business, and isolates from ground, completely. As such it's got a massive iron core that saturates nicely, so it's immune to even the largest of spikes and helps with line noise. The addition of a 4" iron core choke and three stage filter choke finishes it. The way the setup is, my cats could come chew on the power wires (one at a time I suppose) and not get shocked, and I've had lightning hit here numerous times with no problems.
and this:
pmset -g ps | head -n 1 | cut -d "'" -f 2
will either tell you "AC Power" or "UPS Power", making it trivially easy to get cron to drop me an email when it falls over to UPS power.
