Workspace
The calculator's own fields, action and results arrive with the verified pack when you load it. Nothing is computed in this page.
Home efficiency · the bulb swap judged
LED conversion simple payback
Judge a whole-home LED swap in years: the bulbs’ annual saving is computed from count, watts and hours, and the price of the box is divided by it.
Verified engine
What the engine returns
The engine returns the horizon in years with the annual saving beside it. Read the years against the lamps’ service life at those daily hours: the gap between the two is the stretch of pure saving the swap buys, and it is what makes the verdict robust to the ratio’s omissions.
Inputs the result is built from
Total cost of LED bulbs
Bulbs replaced
Watts saved per bulb
Average on-hours per bulb per day
Electricity tariff
How the result is derived
MethodThe bulb count multiplies the watts saved per bulb into a total wattage; the daily hours and the days of the year turn it into kilowatt-hours saved; the tariff prices the kilowatt-hours into an annual saving; and the total spent on bulbs is divided by that annual figure to give the horizon in years.
StandardSimple payback screening: total bulb cost over the annual saving, with the saving by home-energy audit bookkeeping
GuardA zero spend is refused — the pack ships that refusal as a declared test vector — because a free swap has no payback question, and a zeroed field is more often a missing receipt than a gift.
How the payback moves with daily on-hours
When a box of bulbs has paid for itself
This page runs the saving sibling’s whole chain internally and then performs the division, so the verdict can be made from the basket alone: the bulb count and per-bulb watts become a total wattage, the daily hours and the days of the year turn it into kilowatt-hours, the tariff into money, and the total spent on bulbs divided by that money is the horizon in years. The annual saving is returned alongside, so the verdict can be audited against the saving page’s own story.
The cost input is the receipt, not the price per lamp: the whole outlay for however many bulbs the count says. Keeping the two sides of the ratio describing the same estate is the page’s one discipline — a spend for the whole house divided by the saving of one floor invents years, and the reverse invents a bargain.
Simple payback omits discounting, price escalation and the value of better light, but for lighting the omissions argue about very little: the spend is small, the measure is reversible, and a modern lamp’s service life is long in years of ordinary burn. When a horizon computed here comes out short against that service life, the residual decades are simply saving — which is why lighting is so often the first line item an audit recommends.
The same ratio governs every measure in this pack — insulation, sealing, the appliance premium — so the number this page produces is directly comparable with the heavyweight retrofits. Ranking a box of bulbs against a loft quote by years rather than by sticker price is the whole point of pricing the small measure honestly instead of assuming it.
The bulb count multiplies the watts saved per bulb into a total wattage; the daily hours and the days of the year turn it into kilowatt-hours saved; the tariff prices the kilowatt-hours into an annual saving; and the total spent on bulbs is divided by that annual figure to give the horizon in years.
When this calculation is used
- Standing in the lamp aisle: the basket total against the estate it converts, before checkout rather than after.
- Choosing between replacement technologies — the cheaper lamp with the smaller per-bulb saving against the dearer one — by years instead of shelf price.
- Ranking the bulb swap against the household’s other candidate measures, all judged by the same undiscounted ratio.
- Deciding whether to convert the stragglers now or on failure: pricing the incremental basket for the fixtures still burning filament.
Worked example
Judge the pack’s declared anchor swap: a box of bulbs costing 150 all in, converting 30 fixtures at 40 watts saved each, lit for 4 hours a day on average, at a tariff of 0.15 per kilowatt-hour.
The engine returns the horizon in years with the annual saving beside it. Read the years against the lamps’ service life at those daily hours: the gap between the two is the stretch of pure saving the swap buys, and it is what makes the verdict robust to the ratio’s omissions.
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 stress the survey, not the arithmetic: rerun with honest hours and watch what the horizon actually depends on.
What each input represents
Total cost of LED bulbs
The whole receipt for the box — every lamp the count below claims, at what they cost. It is the numerator of the horizon, and it must describe the same estate as the saving inputs beside it.
Bulbs replaced
A whole-number count of the sockets the spend converts — fixtures in real use, not spares. The count must be the one the receipt paid for; the ratio only means something when both sides describe one basket.
Watts saved per bulb
The old lamp’s draw minus the replacement’s at matching light output — a difference, never the old rating alone. The default is illustrative of a filament-to-LED swap; a halogen estate sheds less per socket.
Average on-hours per bulb per day
The daily burn averaged across the counted bulbs, weighted toward the rooms that dominate real use. Optimism here shortens the computed years without shortening the real ones.
Electricity tariff
The marginal per-kilowatt-hour price from the household’s bill; the default is illustrative only. Dearer electricity shortens every horizon on this page.
Assumptions and limits
- The horizon is undiscounted and static: this year’s saving is presumed to repeat identically, with no energy-price escalation and no value on better or worse light.
- The spend is bulbs only, as entered; fixtures that need rewiring, new drivers or an electrician belong in a fuller cost before the ratio is relied on.
- Every lamp in the count is replaced like-for-like in light output and stays in service for the whole horizon — early failures lengthen the real years.
- The estate survey behind the saving inputs is honest; the division cannot repair an optimistic hours figure, it can only propagate it.
What the guards protect against
- A zero spend is refused — the pack ships that refusal as a declared test vector — because a free swap has no payback question, and a zeroed field is more often a missing receipt than a gift.
- A zero per-bulb saving and zero on-hours are each refused before the division: either way there is no saving to divide into, and the horizon would explode toward infinity instead of naming the survey error.
- Daily hours are bounded by the hours a day contains, so a weekly figure entered in the wrong period is refused as a units mistake rather than compressing the computed years.
Provenance
Simple payback screening: total bulb cost over the annual saving, with the saving by home-energy audit bookkeeping
The bulb count multiplies the watts saved per bulb into a total wattage; the daily hours and the days of the year turn it into kilowatt-hours saved; the tariff prices the kilowatt-hours into an annual saving; and the total spent on bulbs is divided by that annual figure to give the horizon in years.
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.