Interesting you should bring this up. I have just bought a device which goes between plug and socket to show how much power something is using.
On Sunday, all day and evening, my laptop running on mains used 0.07 Kw - this is what I use nearly all the time in the house.
Today I used my desktop from around 9.30 until 17.30 as I was backing up some files (thats about all I use it for now) - it is not one of the more powerful desktops, amd XP3000 with 300 watt power supply, fx 5600 graphics and 2 hard drives.
It used 1.53 kw, more than 20 x the power consumption of my laptop