One thing often overlooked is that even if a battery pack has reached the end of it's useful life for a car (i.e. 50% of it's original storage capacity), it still potentially has utility for stationary storage purposes. It seems like Daimler is already working on reusing car batteries in this way: http://arstechnica.com/science/2015/11/daimler-is-reusing-el...

... and I read once that Elon Musk talked about it, though I can't find a link.