Enlarge / Solar panels sit on the roof of Google headquarters in Mountain View. (credit: Kimberly White/Corbis via Getty Images)

Google has long been a carbon-neutral company in a theoretical sense. That is, even when it’s physically impossible for Google’s data centers and offices to consume renewable energy, the company offsets that “dirty” energy with “clean” energy purchases at other times and locations.

The problem is, this does not make Google carbon-neutral in a practical sense, because the company still needs polluting energy sources to keep functioning. In a new report (PDF), Google has acknowledged this limitation and offered a few interesting graphics showing how much carbon-free energy its data centers actually consume.

The report is interesting not just because Google is the largest corporate buyer of renewable energy in the world but also because it shows Google is heading off criticism that has been lobbed at all kinds of corporate buyers of renewable energy, including major players like Facebook and Apple. That is, if you’re “offsetting” your carbon emissions by paying a wind farm owner for energy that’s created at 2am on land that’s 3,000 miles from your data center or factory, how much good have you really done?