Last quarter, Google posted information about the efficiency of Google data centers and promised to update this information every quarter. They have now kept that promise and published the collected data for the fourth quarter of 2008 to sustainable computing website.

Here are the rules:

• The data center has to have been Google designed (many are colocation sites that suffer from much of the same problems as everyone else)
• The site needs an IT load of at least 5 MW
• The site needs to have been in operation for at least 6 months (I guess this is so that Google can actually fill up the site and get a high loading efficiency)

In Q4 Google claim an average power and cooling overhead in these facilities of 16%, bringing the overhead for the trailing 12 months to 19% (down from 21% a quarter earlier). For comparison, a recent EPA report put the overhead of the average enterprise data center at 100% or higher. Google fairly report that a number of factors contributed to the result.

First, efficiency is affected by seasonal weather patterns — cooler weather is better than hot weather, and several of the facilities benefited from that in Q4.

Google also measure and manage the efficiency of their site proactively as discussed below:

“Also, we continually review our efficiency metrics so that we notice, for example, that one of our data centers is not performing consistently with others of similar size and locale. So we’ll take a closer look at optimizing that facility. Are we using fans to cool spaces that don’t need to be cooled? Is the thermostat at the right set-point? Can we reduce the time the chillers need to run while keeping the machines operational? So we apply lessons we’ve learned from better-performing data centers to other facilities, and several such improvements took place in Q4. For the nitty-gritty technical details, visit our data center efficiency page.”

Google say that they will continue to share their data and best practices publicly.

“In early March we will participate in the CeBIT conference where we plan to disclose more details on our sustainability efforts as part of this year’s theme of “Green IT.” Stay tuned.”