Nowadays, cloud federation is paving the way toward new business scenarios in which it is possible to enforce more flexible energy management strategies than in the past. Considering independent cloud providers, each one is exclusively bounded to the specific energy supplier powering its datacenter. The situation radically change if we consider a federation of cloud providers each one powered by both a conventional energy supplier and a renewable energy generator. In such a context the opportune relocation of computational workload among providers can lead to a global energy sustainability policy for the whole federation. In this work, we investigate the advantages, constrains, and issues for the achievement of such a sustainable environment.