I am thinking that you'll need a bit more of a UPS than that model. walk with me as we do some math...
If your 27" tv requires 140W (guess), your computer requires 300W and both the router and cable box require 90W each (all guesses, you may have better numbers...) then you are looking at 620W req'd at any point in time. Already, your UPS choice is too small, look for something bigger than 620W (I think they make 500VA and 750VA models. 1VA (volt-amp) does not equal 1watt, but for this example, they are probably pretty close. Serious EE's will call me out about the capacitive loading of your TV (if its a CRT...), and I'll take that hit, I've forgotten my AC theory.
I cannot find the amp-hour ratings of the batteries on any radioshack page. Looking up a similar model on newegg.com (
surf to here
its shows that a 300W load only lasts 5 minutes. blah. You'll need 6 times the # of batteries this thing has to power your setup for 30 minutes. Bigger UPS's will have more/larger batteries, but you will pay more. Most UPS's are used to provide just enough power to keep the machine alive long enough for you to hit save and shut down (or to clean up the dirty power that plagues most of the world...). Some UPS makers make battery packs that hook to an existing UPS to extend its battery life (you can tell by the big cables they have to use to hook them up together.
If you give up and turn the TV off when power goes out, you'll increase your battery life by almost a factor of 2, with any UPS. But I think you can see where I am going - to keep your TV and computer going during an outage is going to require more than $100 in UPS equipment.
The small UPS you linked would give you enough time to hit save and shut down windows normally (prolly a bit more). But I really don't think that it's gonna last you 30 minutes.
-John