Compared here are the power consumptions of the three current consoles Having graphics from a couple of years ago pays of big time for the Wii here. If all 13 million sold Wii’s would be running they save roughly the power that Hoover Dam in Nevada generates. If you would assume that 13 million PS3/XBox360s or PCs would be running otherwise. This excludes any transportation losses etc.
Even more interesting is the comparison of power consumption for Movie playback. During gameplay you actually get different results from the Wii versus the high performance consoles. But the movies look the same.
As shocking as those graphs look it costs roughly 4 cents more in energy to watch a 2.5 movie on a game console / PC instead of a dedicated player.