Electricity providers report power use in kilowatts hours, since the power consumption of your entire house is going to be large compared to a single PC. Every light bulb, TV, microwave, refrigerator, vacuum cleaner, etc. requires power. Unless you are running a lot of computers, it may not even be necessary to think much about how much your computer uses without addressing those other areas first. Still, there's a large difference between an entry-level PC with EIST/Cool & Quiet sitting at the desktop and a high-end PC running the latest 3D game.

For our comparisons, we will look at two states in the US (North Carolina and California) and Germany will represent Europe. We used an exchange rate of $1.30 per Euro. Power use is calculated by the above chart, factoring in the efficiency of the power supply. For simplicity's sake, we will start by assuming 82% efficiency on all systems and loads. Divide the power consumption by the power supply efficiency and you end up with the actual power use in Watts. Converting Watts into kWh requires a bit more math: take the power draw in Watts and multiply that by the number of hours a device is running, and then divide that number by 1000.

http://www.anandtech.com/casecooling...spx?i=3458&p=1