Window upgrade: the annual saving

What the climate pays for better glass

The engine folds the physics step in, so this page runs from the two rating labels directly: glazed area times the drop in whole-window transmittance gives the conductance reduction, the annual degree-days turn the reduction into heat the house no longer loses, the heating system’s coefficient converts that heat into electricity never purchased, and the tariff prices it. Everything downstream is linear in the drop, which is why the delta sibling exists — to argue about the drop on its own terms before money enters.

Window vocabulary runs opposite to insulation’s, and the reciprocity is worth keeping in view while filling the fields: a U-value is a conductance, the reciprocal of an R-value’s resistance, so the better window is the SMALLER rating and the saving comes from how far the rating falls. The largest falls belong to the oldest glass — the span between glazing generations is far wider on the old side of the ledger than between one modern product and the next — which is why the same invoice buys a transformation on single glazing and a rounding error on a recent double.

From the conductance down, this page walks the identical chain the insulation saving page walks, and the annual figures compare honestly for exactly that reason. What is distinct is what the measure touches beyond conduction: replacement glazing changes the solar gain the coatings admit, cures the sash leakage an old frame carried, and moves the comfort line beside the glass. The degree-day chain counts none of these — they push the true total in both directions — so the figure here is the conduction term of the story, stated cleanly rather than the whole story overstated.

The climate and price inputs deserve the cluster’s usual care: degree-days from the weather service’s climate normals rather than one remembered winter, and the tariff from the household’s own bill, blended toward the heating hours on time-of-use plans. The saving is linear in each, so a lazy figure in either field is a lazy answer everywhere downstream — including the payback sibling this page feeds.

When this calculation is used

• Pricing a replacement-window quote’s annual worth from the two rating labels, with degree-days and tariff looked up for the actual site.
• Testing whether the energy case for new windows survives daylight before comfort and condensation are called on to carry the rest of it.
• Comparing candidate products over the same openings by annual figure rather than by rating alone.
• Feeding the payback sibling: this annual figure is the denominator its years are made of.

Worked example

Run the pack’s declared reference case: a house’s ordinary complement of glazing moving from an ageing double-glazed rating to a modern one, in a mid-severity heating climate at a mid-range residential tariff, with electric-resistance heat converting one-for-one.

The engine returns the annual saving with the conductance reduction and the electricity never purchased beside it. Read the kilowatt-hours against the household’s annual consumption first, then hold the money against the likely cost of glazing work — the tension the payback sibling exists to resolve.

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 move only the existing rating a generation older and watch the saving swell — with glazing, what is being replaced decides more than what replaces it.

What each input represents

Assumptions and limits

• Seasonal loss is proportional to degree-days through one constant conductance in a single-zone house, with solar and internal gains presumed folded into the base-temperature convention the degree-day figure was published against.
• Only conduction is priced: new coatings change what the sun contributes and a replacement cures old sash leakage — real channels that move the true total in both directions, uncounted here.
• The chain prices electrically heated homes at one seasonal coefficient — resistance or heat pump; a combustion-heated house needs its avoided heat priced through fuel and efficiency instead.
• One flat tariff prices every avoided kilowatt-hour, presumed neither escalating nor discounted, and the labelled ratings are presumed delivered as installed.

What the guards protect against

• An upgraded rating at or above the existing one is refused — the pack ships the refusal as a declared test vector — because glazing that conducts no less gives the climate nothing to pay for; swapped ratings, not strange products, are what the refusal usually catches.
• Both transmittances are held to the band real glazing occupies, turning away imperial ratings and resistance figures entered where their reciprocal belongs.
• The degree-day cap sits above any climate people heat homes in — a figure beyond it is usually degree-hours or a Fahrenheit total standing where Celsius degree-days belong — and the tariff floor turns away prices entered in cents as if they were dollars.

Provenance

Degree-day heat-loss estimation: the glazing conductance reduction times heating degree-days, with purchased energy scaled by system efficiency

The glazed area multiplied by the drop in transmittance gives the conductance reduction; multiplied by the annual heating degree-days — with the day-to-hours and kilowatt scaling carried in the relation — it yields the heat kept, divided by the heating system’s coefficient for the electricity never purchased, and priced at the tariff.

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.