The math doesn't add up but the tech is really early and there are some people that just want to be independent. The price of these things will drop slowly, just like the prices are falling with LiFePO4 batteries.
The better deal (than Tesla Powerwall) is the
Lavo 40kWh hydrogen battery for about $35,000. That's a ton of money to outlay though, all so you can be "disconnected", but you could run a modest home on that for almost 3 days, and would only need 2 of them to keep a small EV charged.
The big thing about being connected - if you've sized your system right - is that you can save yourself the connection fee, which for us here is about $1.43 per day. That adds up but not enough to pay for the battery. For 365 days = 521.95, and so 10 years makes it just over $5,000. The battery won't pay itself off for 70 years!
You can't win trying to remain connected and recover the connection fee with solar power credits either. Our provider was paying 11c/kW. On our last bill we generated 718kWh for which we received $79.70 credit and our connection fee was $130.13. For our next bill the gap will be greater as the rate has dropped to $0.07/kW!