To answer the title before even reading: not enough to be a concern. (On average) Pennies to $2 per month to run at full load 24/7.
Edit: Aaaand I was just about right. At $39 per year to run one of the most power hungry cards, that's a measly $3.25 per month.
I don't get why so many people (around me, that is) complain "OMG electricity and water are sooo expensive!" ... Really? My water bill has been a constant $12 a month for the past year, and my monthly electric bill has been $115 at its highest and $65 at its lowest. That's with South Florida heat. If you want a pretty accurate reading of what your PC costs in terms of electricity, grab a load meter and multiply your PC's (or any electrical device's) kilowatt hour calculation by whatever your electric company charges per kilowatt hour. FPL for instance charges a certain rate per kWh below 1,000 kWh, and then a little more for each kWh over 1,000.
http://www.newegg.com/Product/Produc...-005-_-ProductEdited by Stealth Pyros - 2/16/11 at 10:24am