Two rates show up throughout RetrofitROI's engines and are easy to confuse: a loan's APR, which is the cost of borrowing money for a purchase, and a discount rate, which is the rate used to convert future savings into today's dollars for an NPV calculation. They serve different purposes and using the wrong one in the wrong place produces a misleading answer.
Financing changes your actual cash cost
The panel upgrade financing impact engine and the envelope financing comparison engine both show the real total cost of financing a project, principal plus interest, against paying cash upfront. A financed project's total cost is always higher than its cash price; the question is whether that premium is worth the flexibility.
Discounting changes how future savings are valued, not what you pay
A discount rate does not change your project's actual cost. It changes how much weight you give to a dollar saved five or ten years from now versus a dollar saved today, when computing NPV. See payback, NPV, and ROI for the fuller explanation of what that produces.
Comparing financing across project types
The battery financing comparison engine, the water heater financing comparison engine, and the EV charger financing comparison engine each apply the same amortized-loan math to their respective project type, so you can compare financing terms consistently across very different purchases.
A financed project can still be worth it
A loan's interest cost reduces total savings but does not automatically make a project a bad idea. If the financed monthly payment is still comfortably below your avoided monthly energy cost, the project produces positive cash flow every month even while you are paying it off, which is a meaningful threshold distinct from total lifetime savings.
Watch for a rate that looks low but isn't
Some promotional financing offers advertise a low headline rate while bundling in a dealer fee or a shorter term that raises the effective cost. Confirm the actual total finance charge each engine reports, not just the entered APR, before treating a financing offer as automatically favorable.
Choosing a discount rate you can defend
There is no single correct discount rate for every household. A reasonable, defensible choice is what you could otherwise safely earn on the cash, or what you would otherwise pay down in interest-bearing debt. Whatever you choose, use it consistently across comparisons rather than picking a favorable rate for one project and an unfavorable one for the alternative you are weighing it against.
Frequently asked questions
What discount rate should I use for my own decisions?
A common approach is your opportunity cost of capital: what you could otherwise earn or would otherwise pay, such as a savings account rate, an investment return you are confident in, or your mortgage rate if you would otherwise pay down debt instead.
Is a low-APR promotional loan always the better choice?
Usually, if the rate is genuinely low and there are no offsetting fees or a shorter-than-ideal term forcing a high payment. Confirm the total finance charge, not just the advertised rate, before assuming a promotional offer beats paying cash.
Why would financing ever be better than paying cash if it costs interest?
If your cash could otherwise earn a return higher than the loan's interest rate, or if you value keeping liquidity for other needs, financing can be the more rational choice even though it costs more in total interest paid.