Get the loan in 2019, with a generous markup on the product so you don’t notice the interest is baked into the price.

Credit cards were offering 0% interest for new customers for up to nearly 2 years (with good credit)

If I did a project like this, I certainly would max out a deal like that, and prioritize that payout over the duration.

If it was from a bank and 3+ years, I'd love to know how to do that too

Mass Save have various low or no interest loans available for improving energy efficiency and generating clean energy if you qualify for them. I used a Mass Save HEAT Loan about 10y ago to replace my old oil burning furnace with a gas furnace and paid zero interest. The new gas furnace had requirements on its efficiency.

While Mass Save sounds really generous, it's being funded by everyone in their electric bill. So in a way, you've already paid into the program. You might as well get something out by calling up a Mass Save auditor and seeing what you can get out of it.

I assume a government program for batteries, but not sure.