Retrofit simple payback

One ratio for every measure in the estate

The chain-specific payback pages in this pack — the bulb swap, the appliance premium, the sealing and insulation verdicts — each compute their own denominator from their own physics before dividing. This page begins where those chains end: the annual saving arrives as an input, from a sibling page, a contractor’s promise or an audit line, and the installed cost is divided by it. Whatever produces a yearly figure in money, this screen can turn into a horizon.

That generality is bought with a warning the specific pages do not need: this screen audits nothing. A chain-specific page constrains its inputs with physics; here the saving is taken at its word, and an optimistic denominator flows straight through the division into confident-looking years. The discipline is to know where the saving came from — and when it came from a vendor, to rerun their claim through the sibling page that computes it from first principles.

The declared reverse workflow is the screen’s second use, and often its better one: fix the horizon the household will tolerate and the engine solves for the largest spend that meets it. Run before quotes arrive, that turns a tolerable number of years into a budget ceiling for any measure at all — and turns every quote that follows into a distance from defensible rather than a number in a vacuum.

A neighbouring page on this site publishes the same division from the solar pack, as that cluster’s destination — its saving arrives from photovoltaic economics, and its story belongs to panels. The pairing is deliberate, not redundant: the relation is portable across subjects precisely because it contains no physics, so each pack carries its own screen, fed by its own estate, telling its own story. This one belongs to the retrofit measures — bulbs, labels, sealing, insulation — and speaks their language.

The ratio’s omissions are the same wherever it appears: no discounting, no escalation, no comfort or resale value, no maintenance. It is a screen, not an appraisal — its job is to rank candidates and expose the indefensible ones, and a measure whose horizon crowds its own service life has earned a fuller accounting, not a bigger ratio.

When this calculation is used

• Finishing an estimate made elsewhere: an auditor’s report line or a sibling page’s annual figure, divided by the quote in hand.
• Ranking unlike measures on one screen — a box of bulbs, an appliance premium, a sealing quote — by years rather than by sticker price.
• Setting a budget ceiling with the declared reverse workflow: the horizon the household tolerates, solved into the most any quote may cost.
• Sanity-checking a vendor’s payback claim by re-dividing their own cost and saving figures, before checking the saving itself against a chain-specific page.

Worked example

Run the pack’s declared anchor screen: a measure costing 1,000 installed against an annual saving of 200.

The engine returns the horizon in years. Read it against two clocks: the measure’s service life, which decides whether the years are followed by pure saving, and the household’s expected tenure, which decides whether they will be collected at all.

Every figure in this example is produced by the certified engine when the calculator loads, checked against the signed pack’s declared test vectors; nothing on this page stores an answer. Then run the declared reverse: fix the horizon the household will tolerate and read off the ceiling any quote must come in under.

What each input represents

Assumptions and limits

• The horizon is undiscounted and static: this year’s saving is presumed to repeat identically, with no energy-price escalation, no discount rate, and no value on comfort, maintenance or resale.
• The saving entered is honest and annual; the screen performs no physics and cannot catch an optimistic figure the way the chain-specific pages can.
• The cost is the complete installed figure — a partial quote shortens the computed years without shortening the real ones.
• Cost and saving describe the same measure over the same estate; mixing a whole-house cost with a single-room saving invents a horizon.

What the guards protect against

• A zero annual saving is refused — the pack ships that refusal as a declared test vector — because a measure that saves nothing has no horizon, and the division would otherwise explode toward infinity instead of naming the claim for what it is.
• The cost must be positive: a free measure has no payback question, and a zeroed field is more often a missing quote than a gift.
• Both figures are held to bands far beyond any household project, so a total entered in minor currency units or with repeated digits is refused as a data-entry slip rather than silently priced.

Provenance

Simple payback screening: installed cost over annual saving, undiscounted

The installed cost is divided by the annual saving to give the horizon in years; the declared reverse workflow fixes a target horizon and solves for the installed cost that meets it.

Screening and reference material, to be checked against the governing standard and a qualified engineer; not a design determination. The signed pack carries its own citation, which displays from the verified leaf when the calculator loads.