The current plan for the new Apple headquarters calls for 500,000 or more square feet of solar panels, generating at least 5MW of power. That could make it the biggest corporate solar panel installation in the U.S. — but Apple operations take so much power that this will just be supplemental.
The proposed building, which would house 13,000 Apple employees, will run primarily on natural gas. The solar panels, which will cover the majority of the building's roof, are providing backup power. Five megawatts is enough to power a million AppleTVs (which I only just found out were a thing), but not enough to run the operations of the company that produces them.
Still, it's a lot more than Google has managed! (They get 1.6MW of solar. Suck it, Google.) And between the giant solar installation and having its primary power facility on campus, Apple has almost divorced itself from the normal electrical grid, which is pretty cool.