While the Xbox 360 is pretty powerful, it takes a fair bit of power to make it that way. A recent graph is running around showing the cost to run various consoles, and currently the Xbox 360 is the most power hungry machine. A few interesting things on the chart. It looks like the PS2 and Xbox 360 use the same amount of electricity through what is called a trickle charge, or leaked charge. This is power being used even when the console appears to be all the way off.
Another surprising fact on the graph is that the PlayStation 1 actually consumes less power than the GameCube and the Dreamcast. Who would have guessed?
The Xbox 360 does need to constantly take in some power though. How else did you think that it knew to wake up when you pressed the button on the wireless remote? It has to keep its wireless port running.
I still think that the Xbox 360 is a bargain to run, even at $20 in electricity each year.