Re: Backup capacity
Who pays? Microsoft, of course. But it's a silly question. Arguments about CO2 and subsidies aside this kind of arrangement makes a lot of sense: the price of the energy produced by the the plant is fixed for pretty much forever. Yes, that includes some hedging for purchasing from the grid should supply not meet demand, but less so than buying entirely from the grid.
Assuming planned demand is close to expected output this makes the data centre independent of the utilities and avoids possible conflicts with other customers. Wind and solar make small, local power plants financially viable: who would build a coal, gas or nuclear power station with just 55 MW capacity?