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 lighting estate priced
Whole-home LED conversion annual saving
Price a whole-home switch to LED: bulbs replaced, watts saved per bulb and daily hours become kilowatt-hours a year, and the tariff turns them into money.
Verified engine
What the engine returns
The engine returns the annual saving with the kilowatt-hours saved beside it. Read the energy figure against the household’s annual consumption from the bill: the share of the whole bill that was lighting waste is the real finding, and it is the number that decides whether the stragglers are worth a second trip.
Inputs the result is built from
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, through the watts-to-kilowatts conversion, turn that wattage into kilowatt-hours saved; and the tariff prices the kilowatt-hours into the annual saving.
StandardHome-energy audit bookkeeping: wattage saved across an estate, held for the daily hours, becomes kilowatt-hours and then money
GuardZero on-hours is refused — the pack ships that refusal as a declared test vector — because bulbs that are never switched on have no waste to shed, and a zero would read as an answer where there is no habit to price.
How the annual saving moves with the bulb count
Why the easiest retrofit in the house is a multiplication
The arithmetic is the same energy bookkeeping the single-appliance page runs, carried across an estate instead of one machine: the bulb count multiplies the per-bulb saving into a total wattage, the daily hours and the days of the year turn that wattage into kilowatt-hours, and the tariff turns kilowatt-hours into money. Nothing in the chain is exotic — which is exactly why the answer is only as honest as the estate survey behind the inputs.
The per-bulb figure is a DIFFERENCE, not a rating: the old lamp’s draw minus its replacement’s, at like-for-like light output. Lumens are the service being bought; watts are only the price paid for them, and an LED swap changes the price while keeping the service. Entering the old lamp’s whole wattage as the saving pretends the socket goes dark, and it flatters the estate by the replacement’s entire draw.
The hours input is an average over a deeply uneven population. A hallway fixture and a bedside lamp keep different hours; a porch light on a dusk timer keeps different hours again. The relation is linear in every input, so the honest options are two: average with the heavily used rooms weighted properly, or run the wings of the house as separate estates and let the linearity add them.
One boundary is worth naming because it flatters and penalises by season: the watts a filament shed as waste were partly heating the room. In a heated season some of the lighting saving is quietly repaid to the heating system; in a cooled season the saving is a little better than the arithmetic says, because the air conditioner no longer fights the lamps. The relation prices electricity at the meter, not the interaction — a limit to carry, not a flaw to fix.
The bulb count multiplies the watts saved per bulb into a total wattage; the daily hours and the days of the year, through the watts-to-kilowatts conversion, turn that wattage into kilowatt-hours saved; and the tariff prices the kilowatt-hours into the annual saving.
When this calculation is used
- Sizing the prize before buying the box: a walk through the house with a notepad, counted sockets and honest hours, turned into a yearly figure.
- Settling whether the remaining filament and halogen stragglers are worth a dedicated trip — the linearity prices any subset of the estate on its own.
- Comparing lamp aisles: rerunning the estate at the per-bulb saving each replacement technology actually offers.
- Producing the denominator for the payback sibling, or for the pack’s universal payback screen when the spend is already known.
Worked example
Price the pack’s declared anchor estate: 30 bulbs each shedding 40 watts, lit for 4 hours a day on average, at a tariff of 0.15 per kilowatt-hour.
The engine returns the annual saving with the kilowatt-hours saved beside it. Read the energy figure against the household’s annual consumption from the bill: the share of the whole bill that was lighting waste is the real finding, and it is the number that decides whether the stragglers are worth a second trip.
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 use the linearity: halve the count for one floor, or rerun the per-bulb saving for the halogen spots, and the estate prices itself in parts.
What each input represents
Bulbs replaced
A whole-number count of the sockets actually converted — fixtures in real use, not the drawer of spares. The relation is linear in this count, so one floor, one wing or the whole estate can be priced with the same page.
Watts saved per bulb
The old lamp’s draw minus the replacement’s, at matching light output. The default is illustrative of a typical filament-to-LED swap; a halogen or fluorescent estate sheds less per socket, and a mixed estate deserves a weighted figure or separate runs.
Average on-hours per bulb per day
The daily burn averaged across the counted bulbs. This is the input most tempted toward optimism — weight it by the rooms that dominate real use, or split the estate and price the heavy-use fixtures on their own.
Electricity tariff
The marginal per-kilowatt-hour price from the household’s bill; the default is illustrative only. Lighting burns mostly in the evening, so on a time-of-use plan the evening rate is the honest one.
Assumptions and limits
- The daily hours repeat identically through the year; seasonal lighting habits — long winter evenings, bright summer ones — must already be averaged into the figure.
- Every counted bulb is replaced like-for-like in light output, so the watts saved is a pure efficiency gain and not a dimmer house.
- One flat tariff prices every saved kilowatt-hour, with no escalation and no time-of-use split across the burn hours.
- The interaction with heating and cooling is outside the relation: waste heat the filaments contributed in winter, and imposed in summer, is not credited or debited.
What the guards protect against
- Zero on-hours is refused — the pack ships that refusal as a declared test vector — because bulbs that are never switched on have no waste to shed, and a zero would read as an answer where there is no habit to price.
- The per-bulb saving must be positive: a swap that sheds no watts saves nothing, and a zeroed field is a survey gap rather than a lamp.
- The bulb count is a whole number of at least a single socket, capped at a scale beyond any dwelling’s fixture inventory, so a stock-keeping figure entered by mistake is refused before it inflates the estate.
- Daily hours are bounded by the hours a day contains, so a weekly total entered in the wrong period is refused as a units mistake instead of being priced as if it were real.
Provenance
Home-energy audit bookkeeping: wattage saved across an estate, held for the daily hours, becomes kilowatt-hours and then money
The bulb count multiplies the watts saved per bulb into a total wattage; the daily hours and the days of the year, through the watts-to-kilowatts conversion, turn that wattage into kilowatt-hours saved; and the tariff prices the kilowatt-hours into the annual saving.
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.