I have two powerwalls that i mostly cycle every day. Lets say its 30Kwh. If i just sold the power back. i would receive $0.12 * 30 that day so $3.60)
If I buy that power, its at $0.32. So, if I cycle the batteries in full, that's $9.60. So, Potentially I can save $6.00 per day.
The units cost be about $23000 installed, so that's 3833 days to break even or just over 10 years.
The batteries are really not a financial thing. I do enjoy however being self reliant on power and with the 20kw solar, i have gone from power costing about $1500+ per quarter, to about $200 per quarter.
For those interested however, you can't really go off grid with these. For example, say you are using 10A per phase, The powerwall will supply 10A for a phase, 20A back to the grid on that phase, and you will still be using 10A on the other phases. So while technically you are zero, without the grid and other users, it would not work. You would need 3 powerwalls and the assosciated switching equipment to be able to be off grid. If they were to ever get flat, they would be unable to use your solar to re-start too as they need mains to start it all off.