Big Internet companies are quick to tout the energy efficiency of their data centers, but Greenpeace is pressuring them clean up their energy supply, too.

The environmental watchdog group today released a ranking of the big Internet companies based on how dirty the sources of their energy are. The analysis includes a clean-energy index and a coal intensity ranking, with Apple and Facebook using a higher percentage of coal-powered electricity than the U.S. overall.

For the past few years, Greenpeace has been seeking to draw attention to how much energy the computing industry uses and the role that the cloud-computing providers play in that usage. It's estimated that computers use about 1.5 percent to 2 percent of all energy, which is equivalent to other energy-intensive industries.

Equally important is that cloud computing is fueling rapid growth in energy use from giant data centers, with energy growth estimated at 12 percent a year. Internet companies should report their energy usage and seek to influence policy to make the grid cleaner, said Gary Cook, Greenpeace's senior IT policy analyst.

"We weren't looking at how efficient they are. They're all saving money with efficiency and that's all super important, but that's only half of the equation," he said. "There's not a lot of innovation happening outside the fence line, they're mainly buying the cheapest energy off the rack."

Yahoo scores the highest on the clean-energy index in large part because it has located its data centers in places with available hydropower. Google stands out, too, for investing directly in wind farms and a solar power project as part of its effort to reduce its carbon footprint. But Greenpeace knocks Google and others for not disclosing enough about how their energy use breaks down.

Greenpeace ranked Internet companies based on a sampling of recent data center construction projects and estimates over energy use and fuel mix.

Google has been putting its money into renewable energy projects, a move that provides a financial return and boosts the amount of clean energy on the grid.

To come up with the index, Greenpeace took a combination of public and company-supplied information on recent data center projects. Based on industry benchmarks for energy use by the size of facility and the energy mix in a certain area, it came up with an estimate on types of fuel powering recent data centers.

"We're not trying to do a carbon footprint. This shows where they are making investment and how they are voting with their feet," Cook said.

Whether it's because of pressure from outside groups or self interest, Facebook has taken steps to clean up its operations. Last week it published the specifications for how it runs its data centers efficiently and it reportedly installed a 100-kilowatt solar array at its Prineville, Ore., facility. The array, while commercial-scale, will only supply a portion of the power consumed there.

Internet companies have a real vested interest in energy, but from an economic point of view, their main concern is getting adequate and low-cost power. From a policy point of view, though, Internet companies as a whole have not shown the willingness to expend the political capital or have the expertise to influence policymakers on alternatives to fossil fuels.