You will need something like the ONEAC 3KVA unit that has close to a
very pure sine wave. At anything over 1/2 rated UPS power, the sine
wave becomes almost a square wave on anything except the very best UPS
units.

Most of the higher current draw SMPS supplies use the sine wave to
double the voltage through a capacitor, diode, and coil doubler
circuit. If you apply something too far off from a sinewave and you
could be asking for trouble.

You will want to check the amount of distortion and variance from a
true sine wave at the load you are going to pull and check with the
plasma tv manufacture to make sure it is well within specification.