Google has just launched a Web page detailing its annual energy use and carbon footprint impact.
It argues that to provide a user with Google products for a month--not just search, but Google+, Gmail, YouTube and everything else it offers - its servers use less energy per user than a light left on for three hours. And, because it says that it's been a carbon-neutral company since 2007, "even that small amount of energy is offset completely, so the carbon footprint of your life on Google is zero."
There are more details here
It has also produced a study about powering email using the Cloud