The most reliable input into any RetrofitROI engine is not a national average or a contractor's marketing claim, it is your own utility bill and your own quote. Using them well means extracting the right numbers and being honest about what they do and do not tell you.
Decomposing a bill into its real components
The utility bill cost decomposition engine is the starting point for turning a confusing bill layout into the specific rate and usage figures other engines need. See reading a utility bill for the fuller walkthrough of that process.
Isolating a specific end use from a whole-house bill
A whole-house bill does not separate water heating from everything else automatically. The hot water energy cost engine estimates that specific slice using a smaller set of household assumptions, closing a gap a bill alone cannot.
Comparing a real fuel-cost baseline against an electric alternative
The EV versus gas fuel cost engine uses your actual electricity rate and your actual (or a comparable vehicle's) gas fuel economy to produce a real comparison, rather than a generic "EVs cost less to fuel" claim untethered to your specific rates.
Validating an envelope contractor's savings claim
A contractor quoting an insulation or air-sealing project often provides an estimated annual savings figure. The envelope energy savings value engine lets you check that estimate against your own bill history and a more conservative assumption before treating it as confirmed.
Checking an electrification quote's operating-cost claim
The electrification operating cost delta engine runs the same discounted-NPV check against a quoted electrification project's claimed operating-cost savings, which is worth doing before signing on a number you have not independently verified.
The standard to hold every estimate to
Any savings estimate you did not compute yourself from your own bill and your own entered rate should be treated as a claim to verify, not a fact to build a decision on. RetrofitROI's engines exist specifically to let you run that verification in a few minutes before committing real money to a contractor's number.
Frequently asked questions
How many months of bills do I actually need to check a purchase decision?
A full 12 months is the standard for anything seasonal, heating, cooling, or solar production. For a decision unrelated to season, like EV charging cost, a couple of representative months can be enough, but 12 months is never wrong, just occasionally more than strictly necessary.
A contractor gave me an estimated annual savings number. Should I trust it?
Treat it as a hypothesis to verify, not a given. Compare it against what your own bill data and RetrofitROI's engines produce using your actual rates and usage before relying on a contractor's own figure, which has an incentive to look favorable.
What if my usage varies a lot year to year?
Use an average of the last two or three years if available, and note the variance yourself so you understand the uncertainty band around your estimate rather than treating a single year as precisely representative.