Advanced options
What The Light Bulb Cost Calculator Works Out
- Current cost per year: number of bulbs x wattage x hours a day x 365, divided by 1,000 for kilowatt-hours and multiplied by the unit rate. Twenty 42W halogen spots burning three hours a day cost about GBP 240 a year at the current Ofgem price cap rate (26.32p per kWh from October 2026).
- All-LED cost per year: the same sum with every bulb swapped for a 5W LED, which cuts energy per lumen by around 90%. The same twenty bulbs cost about GBP 30 a year.
- Yearly saving and payback: the difference between the two, with the swap cost at GBP 2.50 a bulb divided by the saving to give the payback in months. Whole-house conversion usually pays back inside a year, the rare home improvement with a payback in months and a lifespan in decades.
- Custom current bulb wattage (advanced): enter the wattage printed on the bulbs you actually have, for example 35W or 50W GU10 spots, or 100W incandescents.
- Electricity unit rate (advanced): defaults to the current Ofgem price cap unit rate but is editable for your own tariff.
- LED replacement wattage (advanced): 4W to 5W replaces an old 40W or 60W bulb (470lm to 806lm); 9W to 13W replaces a 100W (1,521lm). Lumens measure brightness, not watts, and 2700K warm white suits living rooms while 4000K suits task areas.
- LED price and rated life (advanced): what each LED costs and its rated hours, typically 15,000 to 25,000. The life card converts the hours into years at your daily usage, and the projection buys a fresh set each time the rated life runs out.
- Saving per bulb and per month (advanced): the yearly saving split per bulb, useful for deciding whether the rarely used cupboard light is worth swapping, and the monthly figure that comes off the bill.
- Saving over the projection period (advanced): the yearly saving multiplied by the years you choose, minus every set of LEDs bought in that time. Non-dimmable LEDs on old dimmer circuits flicker their way to early graves, so pair dimmable bulbs with a trailing-edge dimmer.