LED Lighting Savings Calculator

The maths that sells itself

Twenty 50W halogen downlights running three hours a day consume 1,095 kWh a year. Swap each for a 6W LED and consumption falls to 131 kWh — a saving of 964 kWh, worth £231 at 24p per kWh, against a bulb cost of £80. Payback: 0.35 years, about four months. Then the LEDs keep saving £231 a year for their 15,000–25,000-hour life — 15+ years at three hours a day. Few purchases in the home return 290% in year one; a bag of LED bulbs is one of them. The test case above is not optimistic — it is the standard British kitchen-and-hallway of halogen downlights, and millions of homes still have them.

Why the saving is so large: halogen bulbs are electric heaters that happen to emit light — roughly 90% of their energy becomes heat, 10% light. LEDs invert the ratio. And unlike every other measure on this site, the "installation" is screwing in a bulb. There is no survey, no scaffolding, no installer, no planning permission. If any retrofit deserves the word "no-brainer", it is this one.

Lumens, not watts: how to buy the right LED

Stop shopping by watts — watts measure energy in, not light out. Shop by lumens: 400–500 lumens replaces a 40W incandescent, 700–800 replaces 60W, and 400–550 lumens at a wide beam angle replaces a 50W halogen spotlight (spotlights need fewer lumens than the headline suggests because the light is directed). The packaging shows both; match the lumens of the bulb you are replacing and you will not notice the difference — except on the bill.

Colour temperature is the second number that matters, measured in kelvin: 2700K is warm white (the cosy incandescent glow — right for living rooms and bedrooms), 3000K is slightly crisper (kitchens, bathrooms), 4000K+ is cool/daylight (offices, garages, anywhere you work). The great LED mistake is buying 6000K "daylight" bulbs for a living room and living under motorway-services lighting. Buy 2700K for anywhere you relax. CRI (colour rendering index) is the third: 80+ is fine, 90+ makes colours — food, skin, paint — look natural. Cheap unbranded LEDs sometimes skimp here; the difference is visible.

Dimmers: the compatibility trap

The number-one LED complaint — flickering, buzzing, or refusing to dim low — is a dimmer problem, not a bulb problem. Old leading-edge dimmers were designed for 200W+ of incandescent load; a circuit of six 6W LEDs presents 36W, below the dimmer's minimum, and the electronics misbehave. The fix is an LED-compatible trailing-edge dimmer (£15–£25, a 20-minute swap for anyone comfortable with basic electrics — mains off), plus dimmable-rated LED bulbs. Check the bulb manufacturer's dimmer compatibility list if you want certainty; the major brands publish them. Non-dimmable LEDs on a dimmer circuit will fail early and flicker — match the bulb to the circuit or bypass the dimmer.

Where the remaining halogens hide

By 2026 most UK homes have converted the easy bulbs, but halogens persist in predictable places: kitchen downlights (the 50W GU10 army — the single biggest remaining prize, often 10–20 bulbs in one room), bathroom spots (frequently left because "they're only on briefly" — but there are often six of them), outdoor floodlights (500W halogen security lights are still common and outrageous — a 30W LED floodlight replaces one, saving £100+ a year if it triggers nightly), and oven and cooker-hood bulbs (special high-temperature types; replace like-for-like when they blow). Do a walk-round with the calculator: count every non-LED, enter the totals, and let the payback — usually months — motivate the ladder work.

The heat-replacement footnote (for the thorough)

Pedants — correctly — note that halogen waste heat slightly warms the house in winter, so some of the "saving" is offset by the heating working harder. The effect is real but small: even in winter, only a fraction of bulb heat usefully displaces heating (much of it rises to the ceiling or heats empty rooms), and for half the year the heat is pure waste. The calculator does not deduct it, which overstates winter savings by perhaps 10–20% for heating-season hours — nowhere near enough to change the verdict. In summer, the waste heat is an active penalty: halogens make hot rooms hotter.

Disposal and the long life

LEDs last 15,000–25,000 hours — at three hours a day, 13–22 years. You will likely move house before you replace them, which makes the "£4 each" objection look silly spread over a decade. When they do die, they are electronic waste: take them to the council recycling point or the retailer's take-back scheme, not the general bin (they contain small amounts of electronics and, in some drivers, capacitors). The old halogens and CFLs you remove: halogens go in general waste (they are just glass and filament), but CFLs contain mercury — bag them and take them to hazardous-waste collection. Do not smash a CFL indoors; ventilate the room if you do.

Lighting design: fewer, better lights

The deepest lighting saving is not a bulb swap — it is needing fewer bulbs. British homes are chronically over-lit by accident: a kitchen with sixteen 50W downlights installed by a builder who billed per fitting, when six well-placed LEDs would light it better. When you convert to LED, treat it as a redesign opportunity, not a like-for-like swap. Halve the downlight count and spend the saving on two good pendant or under-cabinet fittings: task lighting where you work (under-cabinet strips in the kitchen, a proper reading lamp) plus low ambient light elsewhere uses a fraction of the energy of grid-pattern ceiling spots blasting every corner equally. A living room lit by three 6W lamps on dimmers (18W total, dimmed to half most evenings) is cosier and cheaper than eight 6W downlights at full blast — and it was the downlights that needed the electrician.

The same logic applies outdoors: a 30W LED floodlight with a PIR sensor that runs minutes a night replaces the 500W halogen dusk-to-dawn unit burning all night "for security" — a saving that can exceed £150 a year on its own, before the bulb-swap maths even starts. Security lighting that is off most of the time is also better security: a light that snaps on is a signal; a light that never goes off is wallpaper. Fewer, better, controlled — that is the whole philosophy in three words.

Smart bulbs: worth it?

Smart bulbs (£10–£25 each versus £4 for a plain LED) add app/voice control, scheduling, and colour. The energy case is nil — a plain LED is already 90% efficient; no app improves on that. The case is convenience and behaviour: scheduled "away" lighting for security, or motion-sensor control ensuring lights are never left on. For most homes, the winning combination is plain LEDs everywhere plus smart control only where it changes behaviour — a motion sensor on the hallway, a schedule on outdoor lights. Do not pay a 4x premium per bulb for features you will use twice.