The trouble I have with Elon Musk is the cult of personality that surrounds him. For example, he didn't become part of Tesla until several years after it was founded. If you want a different take on that company, then take a look at this article...
The Register has a slightly more cynical take on the announcement.
http://www.theregister.co.uk/2015/05/04/tesla_powerwall_the_game_change_flavoured_battery/
The real limit that they identify is the peak power output isn't going to run a house unless there are multiple units installed...