The good thing about Mac's Energy Saver preference panel and the CyberPower UPS is that you can set it up to self shut down if the power goes off. I leave my iMac on 24/7, so I might be out of my house when the power goes off, or just taking a shower. I think you're overlooking how useful it can be when you're not there.
A 42" plasma draws 200-300 watts also. You might be cutting it close.
Also, do you realize that a pure sine wave UPS is probably not that important, unless your intent is to run for a long period of time without power from the Grid.
- All Macs have an Active Power Factor Correction power supply (called PFC), which allows them to be used with a wide range of voltage and electrical quality. I believe this was implemented in 2007
- UPS manufacturers want you to believe that pure sine wave models (more expensive) are necessary for PFC computers. BUT, that's only if you're running for a long period of time (say a server). You have already said you intend to shut down your computer (or as I recommend, set up for auto-shutdown), so we're talking just a few minutes.
- A well designed UPS (which we're discussing, not some off brand piece of junk), the UPS should be able to transfer from external power to battery in a few milliseconds. Mine has the spec of 8 milliseconds. Here's where the problem occurs and where you might have an issue. If you do not have enough "headroom" on your UPS to handle all the wattage being drawn, even a pure sinewave UPS will cause a dropped power load.
So if your intent is just short-term, and if you want to add that plasma, my suggestion is you don't spend money on a pure sine wave UPS, and up the rated wattage of your UPS to handle that instanteous surge where the APC may draw a full load at the moment of switchover.
I went with a non-sinewave (but a simulated one) with 600 watts which is huge overhead to my 300 watt iMac (at it's highest draw), and a small 15 watt draw of an external Time Machine drive.
I tested my system by shutting off the relay to my home office, and I got about 10 minutes of power for both (but the screen was on the iMac, which might be different if it were in display sleep mode). The battery worked flawlessly, though I probably wish I had gone with a 1000 watt version, just in case the shutdown takes a long time.
Don't overpay for what you don't need. If you want to protect all of the devices you want, spend on reserve power, not on a pure sine wave.