A quicky look at the panasonic website shows that a 42" Plasma uses about 122W when operating, and the 42" LCD uses negligibly under 90W when operating.
So about 33% more power (compared to current models).
Compare that to slightly older 42" LCD's which use upto 200W
It's not much different now.
Electricity in the USA is VERY cheap compared to the rest of the world.
It's 18-22c / KWhr here in Oz for consumers. Aussie dollar is practically parity with the USA.
Europe ... it's a tad more expensive.
To be considered is the picture quality (especially contrast), and processing quality, and any compatibility problems with different devices.
I can usually tell the difference between a 720p and 1080p TV .... would it actually affect whether I enjoy what I'm watching ... most likely not .. unless whatever I was watching was already marginal entertainment.
Regardless, these days 1080p is standard, unless you're looking at older models, or TV's less than 40".
I can' only assume that you're looking at a bigger TV than 37", since it's still working fine