IRS publication 936,
Secured Debt
You can deduct your home mortgage interest only if your mortgage is a secured debt. A secured debt is one in which you sign an instrument (such as a mortgage, deed of trust, or land contract) that:
•Makes your ownership in a qualified home security for payment of the debt;
•Provides, in case of default, that your home could satisfy the debt; and
•Is recorded or is otherwise perfected under any state or local law that applies.
If only the solar panels are collateral, and the loan is not recorded at the county clerk's office, then it is not a qualifying mortgage loan for the mortgage interest deduction.
For improvements to rental property, you can deduct interest against rental expenses even if the loan is not secured by the property, as long as you can show a direct connection from the proceeds of the loan to the improvements. You can even deduct credit card interest, ifs you use your credit card to improve your rental property.
But for the personal mortgage interest deduction on your own home, on schedule A, it must be a qualifying secured loan.
