As long as the bond issuer remains solvent. How much do you trust bonds that yield 9% to retain their full value for 25 years?
"Another way to look at this is that the investment is returning ~9%/year."
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.
You reminded me: David Roberts' often states that a hurdle for electrification and decarbonization projects is connecting them to "slow capital". Stuff like residential, community, solar, battery, heat pumps, appliances, ground source heat, yadda yadda.
I gather that there's plenty of "slow capital" perfectly happy with low risk long term modest returns. But these projects are too small to be worth the effort. Probably something about transaction costs.
My impression is there's an opportunity to bundle up these projects for the larger/largest investors. Biden's IRA created a "green bank" (RIP); maybe that was its intended function.
You're smart about money and finance, so you can probably explain what Roberts is talking about (to noobs like me).