Take a look at how much your water heater, A/C, or heating are using. It will be orders of magnitude larger.
I became kind of fanatic in reducing my desktop's power consumption, but after getting a kill-a-watt meter I noticed my electric bills are dominated by my water heater. Unless you have a really crazy desktop or multiple desktops in your home, they aren't using too much.
btw my Desktop uses ~125W idle and has a Core i7 920, a GTX260 graphics card, and 4 7200RPM hard drives. Most laptops use less than 125W when max loaded...
Haven't measured the usage on my new MBP yet (see signature).