An incentive is one of the few inputs that can single-handedly flip a purchase decision from "not worth it" to "worth it," which is exactly why it is also the input most often entered wrong. Getting it right means being specific about what you have actually confirmed, and entering it exactly once.
The default is zero for a reason
Every RetrofitROI incentive field, on the solar incentive impact engine, the water heater incentive impact engine, and elsewhere, defaults to $0. That is deliberate. An unconfirmed incentive assumed into a payback calculation produces a number that looks better than your real situation, and you only find out it was wrong after you have already committed to the purchase.
The most common double-counting mistake
It happens when a homeowner enters a rebate into the incentive field and also lowers the installed-cost field to reflect the same rebate, effectively applying it twice. RetrofitROI's engines always subtract confirmed incentives from the installed cost you enter internally, so enter your full, pre-incentive installed cost in the cost field and the incentive amount, separately, in the incentive field.
Stacking incentives correctly
A federal tax credit, a state rebate, and a utility program can sometimes all apply to the same project, but each has its own rules about whether it stacks with the others and whether it is capped as a percentage or a flat dollar amount. Confirm the actual combined dollar figure with each program's current documentation, then enter that single combined total. RetrofitROI's incentive fields are capped internally so no combination of entries can exceed your installed cost.
Eligibility is not the same as availability
A program existing and you qualifying for it are two different facts. Income limits, home age, equipment efficiency tiers, and whether you own versus rent can all affect eligibility. RetrofitROI does not verify eligibility for you, on purpose: that is a determination your installer, tax preparer, or the program administrator has to make, not a website.
How an incentive interacts with financing
If you are financing the purchase, check whether the incentive reduces the amount you finance or arrives later as a rebate check or tax credit after the loan is already funded. Those are different cash-flow timelines. The electrification incentive impact engine and the EV charging incentive impact engine both assume the incentive reduces net cost immediately; if yours arrives later as a tax credit, you are effectively financing that gap in the meantime.
A worked example of the mistake
Consider a $10,000 heat pump with a genuine $2,000 confirmed rebate. Entered correctly: installed cost $10,000, confirmed incentives $2,000, net cost $8,000. Entered wrong, with the installed-cost field already reduced to $8,000 and the incentive field also set to $2,000, the engine would compute a net cost of $6,000, understating your real cost by $2,000 and overstating both payback speed and NPV.
When you do not have a confirmed number yet
Run the calculation at $0 first to see the true unsubsidized picture. Then run it again with a conservative estimate you have reason to believe, clearly labeled in your own notes as unconfirmed, so you never mistake a planning guess for a locked-in number when you are ready to sign a contract. The heat pump vs. gas furnace comparison is a good example of a decision where the confirmed-versus-assumed distinction routinely changes the winner.
Frequently asked questions
What counts as a "confirmed" incentive?
One you have actually verified applies to you: a rebate your utility has approved, a tax credit you have confirmed you qualify for given your tax situation, or a manufacturer rebate listed on the current, dated program page. A number a salesperson quoted verbally does not count until you have checked it yourself.
Why does RetrofitROI default every incentive field to zero?
Because assuming an incentive you have not confirmed inflates every downstream number, payback, NPV, and the comparison against alternatives. Zero is the only default that cannot mislead you; you add the real number once you have it.
Can I stack a federal tax credit and a utility rebate on the same project?
Often yes, but not always, and not always on the full project cost. Check each program's own rules for stacking limits before entering both, and enter the combined total as a single confirmed-incentives figure.