I'm no electrical engineer and don't really understand all of the numbers, but in my previous work experiences I've seen a few things about backup supplies, UPS as typically known. I probably won't directly answer your question, but maybe this will help you or others some in determining what you're looking for.
As a general rule of thumb, you get what you pay for. There's an obvious difference between WalMart's 5 minute UPS for $50, and the Hospital Grade 30 Minute UPS for $5000. But even amongst common brands, similar power ratings and all, if you see a price difference of $50 or so there's typically a quality difference.
At work, we had real trouble with Tripp-Lite supplies- the batteries on them would head south in a relatively short period of time for what we paid for them. The general preference was APC supplies. The only other "big name" of supplies that I recall was Belkin, and I don't have any positive or negative impressions of them.
Some supplies had serial cable connections to allow you to setup options, allow for auto-machine shutdown, and allow you to diagnose fault codes. Some even had an option to check battery health- way cool. For whatever reason, as I recall, UPSs with those ports were higher quality. Obviously you'll pay more for the serial port option, but ignoring that factor these were less likely to die.
The final real observation I have kind of addresses your question: How does the UPS handle overload?? Some UPSs instantly shut down the instant they detect overload, even if they have battery left over. Some cut port by port off until back within rating. Others try to fill it until they burn out. It seems to be a potshoot on what you get. The cheaper they are, the less intelligently they seem to handle it.
The most irritating things about UPSs I remember was that when they failed, they didn't quit holding a charge- they died on the spot and broke the circuit. It was so irritating, because we kept our switches and router on UPSs in their closets around the campus. When the UPS died, it took the device with it even though there was power. It woulda been much more preferable to have them just power down on outage then go out while with power. So, you might wanna be ready for that.
If I had to recommend, I'd say go for something obviously over the rating you're looking at. Depending on the time of backup that you're wanting, you should be able to find something relatively reasonable while well over the power requirements. I see an APC BE500C (500VA/300W, 6 outlets, not clear on the runtime) on Pricewatch for $52.95 max cost. I imagine that's at the low end of what you're looking for, but hopefully it's a start for you.
I know I didn't answer your question directly, but I hope this helps some.
