Assuming Folding@home is always running when your computer is on, then it's a matter of power supply wattage*amount of time Folding@home runs.
If you have a 1000 watt power supply (assuming it doesn't go into a stand-by or power saver mode), you are always drawing 1000 watts from your wall when you are running your computer. Your computer may not use all 1000 watts, but it's still drawing that much into the PSU.