What do you mean by "best"? You can get an unsecured personal loan, you can do a cash out refinance (to tie it to your home loan), you can get a home equity line of credit, you can get a specific "solar / PV loan"....
The solar / pv loans that I have seen have this repayment schedule where they build into the pricing you sending your entire federal tax credit in as a principle reduction payment, due by 18 months after the start of the loan, to keep the payment the same as when you started the loan. If you dont send that federal tax credit in, your payment escalates to a higher number.
Tesla uses sunlight financial and mosaic I believe (they sent me to sunlight financial when I was considering getting a loan, before I decided to pay it off).
The interest rate on any of the above is not as low as it is for home loans or car loans right now though.