How do you measure the depreciation? The panels deliver at least 70%, but probably closer to 80%, output after 25 years. The batteries need replacing after maybe 15 years. Assuming that knocks a couple percent points off the return (batteries can only get cheaper and cheaper) that's still a solid 7% long term yield with no default risk. Share your math if you disagree.

EDIT: Two more things that will juice the return

1. Grid electricity prices will go up over those 25 years, at the very least tracking inflation.

2. Unlike bond coupon payments, the "return" from a solar installation isn't taxable. Because you're saving money, not getting paid.