The manufacturer's plate will tell you how many watts the tv draws. A good-size electric space heater draws about 1500-1800 watts. My 42" Sharp TV draws 247 watts. Plasmas IIRC use more electricity.
Casey

Plasma power consumption varies depending on how bright is the image being displayed and the mix of colors involved (which and how many pixels are firing). My 42" Panasonic (6.5 years old) pulls 165 watts with a totally black screen, 245 watts with a blue screen, and up to 425 or so for brighter white. LCD is more constant since the backlight stays fully lit continuously.

Thanks for enhancing my understanding about plasmas. So, on average probably the same, but the maximum power use of plasma can theoretically be higher. It will only matter on a plasma so large that it draws more peak amps than a shared circuit would allow, in which case it would require a dedicated one, provided they ever get that large. (which they won't, since plasmas are being phased out.)
Casey

I highly doubt you will have a problem with the circuit breaker. I have a 50" Plasma connected with my home stereo, xbox, DVD player, HDTV receiver, and computer all on the same circuit. Granted there are only 3 items running at once, but i've never had a trip even while watching movies at a loud setting.

Keep in mind these power consumption figures are taken in the dynamic (torch) mode with 100% contrast which no one uses anyway , but right now plasma's use about 25% more power than LCD flat panel . The power companies were granted a rate increase last year , along with using more electicity in a cold winter both contribute to a higher bill . Plasmas aren't going anywhere , in fact Panasonic is coming out with new models that use less power that are about one inch thick . I have both techs and plasma kicks LCD's butt on PQ with better blacks/shadow detail , no motion blur , viewing from any angle with no picture degradation , and soon will use less power than LCD .

Here is a link that might be useful: New Panasonic Plasmas

The figures I reported above were read from my plasma with a Kill-a-Watt meter, and it's definitely not in torch mode. I've adjusted it properly with Avia.

I understand an LCD TV will use less electricity after it has been properly calibrated.
On the Crutchfield web site, a Samsung 52 inch LCD uses 280 watts according to Samsung. Crutchfield tested it and they measured about 220 watts. That's with the Default picture setting for home use. Standby power use was 1 watt or less.
They estimated a monthly cost of about $4 a month with 6 hours of use and rates of 10 cents per KW.