No one can make batteries that cost less than the mains electricity they can charge and discharge in their lifetime so you can't even break even when the electricity to charge them is free.
Musk isn't going to change that his batteries are bloody expensive. The only way it is going to change is if the price of mains electricity is artificially jacked up much more than it already is for political reasons.
If you could economically store mains electricity in batteries the generators would already be doing it and they would be doing it on a large scale much more economically than stupid little 'home' installations.
Musk's home batteries like his cars are for technically illiterate green willy wavers.