Wat... wut???!? No. You can't power the PC directly from the battery 100% of the time, otherwise you'll drain the battery and have no more power. For the eddification of the populace, 3 types of UPS systems:
Offline: These are the cheapest of the cheap, meant for nothing more than saving your work and turning off. What you have on the input is what comes out the output, with a cursory amount of surge protection. When you have a power blip of any sort, it cuts over to battery. Save quick and shut down until the power gets back to normal.
Line-Interactive / AVR: Better than an offline, and able to adjust for brown-outs or surges without changing over to battery power. What I'd recommend for most home users, but again, save and shut down if it looks like the power's going to be wonky for more than just a few minutes. Business-type units can have extra batteries slapped on to keep their servers running for an hour or two before shutting down.
Online: Bridge-rectifier front end, capacitor-backed internal DC bus, sinewave inverter output. This is the high-end system meant for serious business and network gear. You can almost always strap extra batteries to these, and they're also generator-compatible (unlike the offline and LI/AVR systems, which see the frequency wiggle as bad power and cut to battery).