The problem with UPS's, is they are really only designed to run for a few mins after a power cut, to give you time to do a structured shut down of a system, and obviously to cover brown outs, short off/on's of power and surges.
The longer you need them to run for, or for higher current loads, the price shoots up. The batteries only really last 2 years or 3 at a stretch, but then could well fail before the calculated cover time, regardless of how many power cuts you have in their life, and as they tend to be specific sized Sealed Lead Acid, can be quite expensive to replace. A lot of the more affordable UPS's use a cluster of NiCads and those can be practically impossible to source for a price that makes them worth replacing.
I have never done the calculations, but I think you would probably save money getting a smart battery charger, a couple of deep discharge batteries, and an inverter.
By the way Zac, you keep talking about the potential security issues of Cat5/6, but when I worked at the University, we had a whole RJ59 network taken down with a well placed pin through the cable... It also became the preferred way to silence nuisance CB'ers in the 80's as well, as it would short the output, and blow their CB output transistor...