I basically pay for my batteries with a HELOC I took out for my home energy project (solar equipment, added insulation, converted my two natural gas appliances to electric ones that are very efficient appliances). My HELOC payment plus small power bill were a hair more each month than what I was paying in year 2019 for power + natural gas (year 2019 IMHO is a good standard for energy prices since covid hadn't distorted energy prices). And the solar tax credit paid down a significant chunk of my HELOC, which made my HELOC payments go down to the point where my HELOC payment + power bill in year 2022 was even to what what I was paying for power + natural gas in year 2019. Yay! I had beaten 3 years of inflation! Further, if I stayed the course, as I slowly paid down the HELOC my HELOC payments would go down in future years, while the energy prices I avoided half of would go up. In other words, it's cost me less to save more, then the next year it'd cost me even less to save even more, etc. And this was with Phase I of my solar project, producing 58.5% of all the power we consumed in a year. But that wasn't as far as I wanted to take it if Phase I proved that the equipment and my analysis of solar in my area plus mine and my wife's power consumption habits were correct. So I did the data analysis (yay to my inverter having a good telemetry export recorded at 5-minute candles) and implemented Phase II last year. That included replacing my wife's car (which needed replacing anyway) with an EV, doubling my solar panels and inverter capacity, and tripling my battery storage. Last month was the 1-year anniversary of the upgrade and it's producing 82% of all the power we consume in a year, including charging the EV (by my estimate about 22K to 23K miles are charged at home of the total 26K miles we drove the first year).
So far the overall energy project has saved my cash flow (read: my retirement investments) a total of $1,900 more than it's cost me (expenses I didn't put into the HELOC). Now that the EV is part of the equation, I'm basing my gas and oil change savings off of 2019 prices too. Any time the overall energy costs ($75 power bill + HELOC payment + car payment) are more than what I was paying in year 2019 for power + natural gas + gasoline + monthly savings deposit for car repairs and replacement I take the difference from the HELOC. And when I get the tax credits (which I get 3 tax years for the EV and solar upgrade because the EV and solar tax credits are non-refundable, but the solar tax credit at least carries forward to future years) I use it to pay down the HELOC. At the point the EV is paid off 3 years from now I still will have saved my cash flow $6K more than the project cost me (assuming a reasonable 3% inflation rate of energy costs I'm avoiding in power, natural gas, and gasoline). After that, with no car payment, the savings skyrocket with my energy portion of my budget paying just a small power bill and an ever decreasing HELOC payment.
Admittedly, this was when we could get a loan with a low fixed interest rate.