I recently attended a conference devoted to the theme of energy-efficiency, organized by the Flemish research group Vito.
One presentation, by Dries Vos, a member of the Vito research team, was particularly of interest. Based on a combination of internationally available data and Vito's own research, he came to some staggering conclusions:
Conclusion: if all internal datacenters and server rooms were outsourced to efficiently managed external datacenters, this could lead to an energy saving of about 200 GWh/year, enough to power almost 60.000 families, equivalent to a city almost as big as Liege.
You don't have to take it from me, take it from a renowned research organisation.