Amazon Web Services has been under fire in recent weeks from a group of activist customers who are calling for the company to be more transparent in its usage of renewable energy. [See also: Green IT, renewable energy and sustainability - Greenpeace IT chief puts pressure on AWS]

In response, rather than divulge additional details about the source of power for its massive cloud infrastructure, the company has argued that using the cloud is much more energy efficient than customers powering their own data centre operations.

But the whole discussion has raised the question: How green is the cloud?

In early June a pact of 19 AWS customers including Hootsuite, and Tumblr - wrote to Amazon Senior Vice President Andy Jassy requesting increased transparency in the company's efforts to use clean energy.

The letter was in response to a report from environmental activist group Greenpeace, which singled out Amazon Web Services, saying "no company could do more" to help tech companies be more energy friendly than AWS. The company's cloud platform hosts so many popular websites that any steps it takes to increase efficiency would benefit many other companies.

"Amazon Web Services is holding many of our favourite sites hostage to dirty energy," the report notes. Specifically, it says AWS's US East region, located in Virginia, houses 60% of the company's servers and uses a mix of about one-third coal, one-third nuclear, one-fifth gas and only 2% renewable energy.

In response, the next week AWS announced plans to build an 80 megawatt solar farm in Virginia. Company officials are on the defensive again this week, releasing figures saying that overall, its cloud platform runs on 25% renewable energy, with a goal of using 40% renewable energy by 2016, and eventually 100% green power.

Greenpeace says that's not enough. "It remains impossible for its customers or the public to benchmark any progress toward that goal, since the company refuses to disclose any of its energy data," the report states.

AWS officials argue that the simple fact that so many customers use the company's cloud is saving energy. AWS is more efficient at running data centers compared to its customers, even if it uses fossil fuels to power those data centres, AWS Distinguished Engineer James Hamilton contends.

AWS says customers use 77% fewer servers and 84% less power by running their workloads in its cloud compared to their own data centres. That creates an 88% reduction in carbon emissions for customers who use Amazon's cloud, AWS Evangelist Jeff Barr's says.

Furthermore, the company's US-West location in Oregon, its EU region in Frankfurt and its GovCloud region in the US are what the company calls "carbon-neutral" which refers to the practice of offsetting the amount of carbon the site is responsible for with the purchase of a corresponding number of carbon credits that fund green projects. And AWS is building a 150 megawatt wind farm in Indiana.

AWS isn't alone in having work to do to become more environmentally-friendly. Competitor Google received higher grades from Greenpeace the report gives Google a grade of B, while AWS got a D. Google has also committed to using 100% renewable energy too, although with no specific timeline. Google says about 35% of its operations are currently powered from green sources.

Microsoft, meanwhile stands somewhere in the middle between AWS and Google, receiving a C grade from Greenpeace. The company has committed to being 100% carbon-neutral.

"We know that 100% renewable energy is an ambitious goal that won't be possible overnight," the group of Amazon customers wrote. "While you pursue this journey, we would suggest some steps that will give us full confidence in AWS' commitment to renewable energy." Now there is more pressure than ever for the cloud to be green.