I like this idea... Tesla are looking to turn their batteries into an uninterruptible power supply (UPS). They're putting them in a cabinet that can stand in your garage and will keep your house powered in the event of a blackout.
It isn't a very big jump to realise that (if you have a Tesla) there's something else standing in your garage that could be an emergency UPS too.
Energy supply is a big business and a key market for Tesla, which has spent a fortune developing batteries for its cars. Home batteries can be powered up overnight, when energy companies typically charge less for electricity, and turned on during the day to power a home. They can also be used to store power generated by green - but intermittent - sources like solar and wind.