When I asked Nathan Ensmenger (an informatics professor at Indiana University currently working on a book on the environmental history of computing) about PUE, he offered a pretty blunt assessment. “PUE is [an] atrocious metric. It ignores everything that’s interesting and problematic—basically, if you turn the lights off, you're operating at a hundred percent efficiency.”

PUE only looks to the internal operations of the data center—not, say, where its energy comes from or how much energy they’re using and what that looks like proportionately. Admittedly, accessing some of those metrics is tricky—while Google is happy to report that its data centers account for a scant 0.01 percent of global electricity use, what does that mean in actual kilowatt-hours?

Here’s where things get a bit back-of-the-napkin: According to the U.S. Energy Information Administration, in 2012 global electricity consumption was 19,710 billion kilowatt-hours. Using Google’s 0.01 percent estimate and electricity-consumption data from the CIA World Factbook, they’re using about as much electricity annually as the entire country of Turkey. (Honestly, that number seems impossibly high considering that in 2011 Google disclosed that it used merely 260 million watts of power, at the time noted for being slightly more than the entire electricity consumption of Salt Lake City.) In its 2013 sustainability report, Facebook stated its data centers used 986 million kilowatt-hours of electricity—around the same amount consumed by Burkina Faso in 2012.

Those are some of the metrics used by Greenpeace in their series of reports and campaigns around the environmental impact of data centers. They’ve had a pretty significant role in changing discourse around this issue since they began campaigning around it in 2010.

“When we first started highlighting [Facebook's] data centers running on 50 to 60 percent coal they were saying, ‘Oh that's not fair to say we’re powered by coal, we just buy electricity off the grid like everyone else. But we're really really efficient, why don't you focus on how efficient we are instead of talking about this coal stuff?’” said Gary Cook, a senior IT analyst at Greenpeace. But the company “eventually came around to agreeing that they have a responsibility and an opportunity to drive growth with renewable energy and prioritize access to renewables as they continue to grow.” In 2011, Facebook became one of the first major tech companies to commit to operating on entirely renewable energy, in no small part because of Greenpeace’s campaigning.

Of course, the decision to use renewable energy is as much a business calculation as it is a public-relations one. Tech companies aren’t underwriting wind turbines for their data centers or embracing renewable energy solely out of some altruistic concern for the environment or fear of bad press. If that were the case, they'd be using their billions in profit to build wind turbines to power the grids of entire cities, not just canceling out their own environmental impact.