'APC Domestic' UPS's don't re-power on mains restoration. A USB interface connected to your NAS/PC gracefully shuts down on mains failure.
*I have not reviewed this recently...there may be other auto restore models avaialable.
Some of the small APC UPSes I've used (this was a while ago now) did restore the load when power returned (or it was configurable, I forget). But what you actually want is for the UPS to sit and charge its battery for a bit *before* restoring the load, otherwise when the power inevitably goes off again a minute or two later, there's no charge left for a clean shutdown. DAHIKT.
In practice, I found that small APCs would happily sit and cook their battery for a couple of years, then cause a random weekly outage by going on battery as a test, and failing.
I don't know about the data centre scale stuff, that's always been something that other people have been paid to worry about.
