Benchmarking an Energy Investment I am currently working on a calculation to see whether it is economically feasible for a household to invest in an energy retrofit. I obtain a total profit of around $13\%$ over the lifetime of the investement ($25$ years), which translates to an average annual rate of return of $0.52\%$. This investment is mostly risk-free, to the extent that heating fuel prices remain stable.

What are example of benchmarks that I could use to evaluate this return rate? That is, to make a case for/against the household investing in the energy retrofit?
At its simplest, you just look at the cost of capital: if the household would have to borrow to pay the debt, then what's the borrowing rate? If they are currently saving, what interest rate would they forego by spending on the retrofit instead of saving?

The benefits of retrofit usually extend beyond the simple savings on energy bills, though. When it's done well, it also makes the home more comfortable and more healthy; and sometimes reduce noise from outside too. These things have value, but that value can be harder to quantify. Nevertheless, the value is certainly greater than zero, so just assuming zero because it's difficult to get a decent number, leads to the business case for retrofit being systematically under-valued.