RTFA: http://uclue.com/index.php?xq=724

I have read through the comments, and references, including those provided
by Koomey (and Slashdot!), and I am happy to say that David Sarokin’s
original answer still comes out on top. Here is how I would phrase the
results;

The best estimate of how much electricity the global internet uses (in
2007) is 868 billion kwhs, or 5.3% of world electricity usage. Given the
many unmeasured variables in this calculation, the actual answer might be
less, more, or exactly the same. Until a more current estimate is compiled,
this estimate remains the best estimate. If you have a more current or
differently calculated estimate, I will be most happy to seriously
entertain it. Simply suggesting that this estimate is incorrect won’t
displace it as the best estimate we have.