used to sell ups devices so if you dont wanna struggle with all kinda tech stuff go here and type in how long you wanna watch tv etc and itll popup the right ups :
http://apc.com/template/size/express...emp_country=US
ive looked up a 42" philips plasma, it takes 595 Watt at 95-264 VAC so 30 mins (plus 30% overcap) will cost $878
Mind you : you need to check your plasma tv if it has a start up current thats higher, the ups needs to be able to cope with this start up peak (normally only engine driven devices have this like vacuumcleaner, blowdryers etc but im not sure about a plasma), also : and thats REALLY important, UPS devices generate a different form of voltage then your wall outlet does (has to do with the waveform kinda technical), you need to buy one that has a 100% sine waveform, theyre way more expensive but cheaper ones wont start up your plasma. the one from $878 has a sine wav :
http://apc.com/resource/include/tech..._sku=SUA1000XL