It seems odd not to mention that Musk has an ownership stake in Solar City; they also (as far as I know) have not been doing terribly well as of late - their stock is down quite a bit from its all time high - and the optics of this look pretty bad (billionaire bails out own company using other publicly owned company). This is not to say anything untoward is happening, but from my unimportant perspective, I don't see the value in dropping 2+ billion on this when they've got some huge challenges that already put them in do-or-die mode regarding the Model 3 (yes, it could also be a stock deal).
