Rich Miller at Data Center Knowledge has a good rundown of how Greenpeace's math is a bit wonky in a report called "How Clean is Your Cloud?" For one, Greenpeace estimates Apple will use 100 megawatts of power at the Maiden, NC plant, but Apple states it will be 20 megawatts at full capacity.
Miller later responds in the comments to another mystery that Greenpeace's Gary Cook (no relation to Tim, we're certain) has yet to answer:
"If Apple's energy use in Maiden is adjusted from 100 megawatts to 20 megawatts, what would Apple's score be on the Clean Energy Index? I have posed this question to Gary Cook, but have not yet received a response."
It would appear the fundamental suppositions presented in Greenpeace's report may be flawed, and have skewed their index. I would also ask the question: What would Greenpeace have Apple do? Close the plant? The fact that they are making an investment in clean energy but must face the realities of economics and science seems like good corporate citizenship to me.