The FACE uses durable, unmanned helium-filled blimps with a capacity of 65,536 small EC2 instances, or a proportionate number of larger instances. The top of each blimp is coated in polycrystalline solar cells which supply approximately 40% of the power needed by the servers and the on-board navigation, communication, and defense systems. The remainder of the power is produced by clean, efficient solid oxide fuel cells. There's enough fuel onboard to last about a month under normal operating conditions. Waste heat from the fuel cells and from the servers is used to generate additional lift.
From Amazon Web Services Blog - Link