I've looked into the ROI of batteries and solar and I haven't found something as nice as an online calculator, but a search for "Powerwall Return on Investment" does pull up several blogs and studies about the topic such as this paper on the Stanford website.
We will now calculate the yearly cost of electricity for a family consuming 20 kWh Off-Peak, 0 kWh Peak, and 10 kWh Partial Peak for the entire year for each state average, A.
Cost Per Day = (20 kWh × (1 -.202) + 0 kWh × (1 + 0.819) + 10 kWh × (1 + 0.202)) ×
= 27.98 kWh × A
Cost Per Year = 365 days/year × Cost Per Day
This means that in California, the cost Per year using our PowerWall 2 strategy would be $0.188 kWh × 27.98 kWh day × 365 days year = $1919.98 year.
That's the type of calculations that you need to do. From what I've read you're looking at anywhere from 8-12 years to recoup the investment on this or a solar install.
My personal opinion is that this is a cool thing to do as a hobby and it may eventually pay for itself, but there are so many better ways to "invest" money. Look at typical 10 year returns on any other $7k investment. Another thing to keep in mind as you read is that most of these "studies" ignore battery degradation to simplify the calculations and they assume electric prices will be constant for 10 years (ha!). And then what about the install costs? And what if anything needs to be replaced or diagnosed?