Interesting:
Apple is also looking to decrease its reliance on electricity provided by the city power grid by building its on-site Central Plant.
What are the benefits of having a dedicated power plant I wonder?
Luckily, it also appears to be possible to use the excess heat for cooling stuff. That would be much more popular in Cupertino, I guess.
Given that and the scale of this thing, they probably think it is cheaper to run those generators permanently. Of course they would still need some extra generators to allow for maintenance and defects, but I guess that's peanuts compared to the startup cost of having the generators for the entire building.