Country Selection Page (http://www.apc.com) should have some info for sizing the YPS in terms of run-time.
Things like POE switches may not have all ports used and therefore will not draw at the specifications.
You should also look at UPS's communication abilities, so that server shutdown can proceed in an orderly fashion.
UPS's served by a stand-by generator require special considerations since not all UPS's are prepared to handle frequency variations.
One of the other decisions you will need to make is wheter the UPS is an always on variety or one that switches to back-up power only when needed. The former is slightly better for power protection.
There is a transformer-like device made by OneAC that I have used and had virtually no power related issues on a single computer for 12 years. It was about $1000 for 1000 VA or 1000 Watts.