The claim of "1 ha" stumped me, and I looked it up - the numbers you can find on the interwebs are conflicting, but it should take up about 200-500 ha. The assumption of 15% efficiency also seems quite conservative to me - I can imagine that they will use new cells fresh from the lab that are more in the 18-20% range. Another thing that Elon mentioned is that they might be able to drive energy consumption down because of synergy effects.

But yeah, they will probably not produce more than 20% of their needs with these solar panels.