An interesting report on Kotaku points out that gamers spend a lot more money on their electricity bill then they do on anything else - and that includes hardware, software, and various services such as Xbox Live, or Netflix. You could probably throw together all that stuff and the cost of power would still be substantially more.
A recently published study from researchers at Carnegie Mellon University reveals that most of the power consumption related to consoles has to do with "idle consoles" left on. One would think it would have to do with consoles being in a "vampire" state (I.E. being off but still consuming power while they are plugged in) or during active usage, but apparently that's not the case.
According to researchers, 68 percent of all game console power consumption in 2010 occurred while in an idle state, which equaled 10.8 TWh of energy and about $1.24 billion in electricity costs. That's a pretty big number. Overall, around 1 percent of U.S. residential energy consumption in 2010 was spent on video game consoles, an increase of almost 50 percent over three years ago.
The Kotaku report drills further into the data from the report, but the take-away is that console makers are trying to reduce power consumption with newer models. Obviously the Wii used the least amount of power.
At the end of the day the take-away lesson for gamers is simple enough: turn your consoles off if you are not using them.