Bulbs used to be sold in watts and everyone knew what a 60W bulb looked like. LEDs broke that shorthand, because a modern bulb produces the same light for a tenth of the power. Three numbers replaced it, and all three are on the box.
Lumens: how much light
Lumens measure the light a bulb emits. This is the number that replaced watts.
Rough equivalents to the old incandescent bulbs:
| Old bulb | Lumens |
|---|---|
| 25W | ~250 lm |
| 40W | ~450 lm |
| 60W | ~800 lm |
| 75W | ~1,100 lm |
| 100W | ~1,600 lm |
If you are replacing a 60W bulb and want the same brightness, look for about 800 lumens. Watts on an LED box now tell you what it costs to run, not how bright it is — a 9W LED and a 60W incandescent produce similar light.
Kelvin: the colour of the light
Colour temperature is measured in kelvin, and the scale runs counter-intuitively: lower is warmer.
- 2,200–2,400K — very warm, amber. Decorative, candle-like.
- 2,700K — warm white. Matches an old incandescent bulb. Living rooms, bedrooms.
- 3,000K — soft white. Slightly crisper. Kitchens, bathrooms.
- 4,000K — neutral / cool white. Workspaces, garages, task lighting.
- 5,000–6,500K — daylight. Blue-ish and clinical. Workshops, and not somewhere you relax.
The most common mistake in a home is putting 4,000K or higher in a living room. It is not that the bulb is faulty; it is that the light is the colour of an office.
Mixing colour temperatures in one room is visible and looks like a mistake. If you replace one bulb in a fitting of three, replace all three.
CRI: how honest the colours look
Colour Rendering Index scores how accurately a light source shows colour, against daylight at 100.
- Below 80 — noticeably flat. Skin looks grey, food looks unappetising.
- 80–90 — the normal range for household LEDs. Fine for most rooms.
- 90+ — worth paying for in kitchens, bathrooms and anywhere you choose clothes or look at your own face.
CRI is the specification most often left off cheap bulbs, and its absence usually means it is not good.
Getting the fitting right
Two things have to match before anything else matters:
The cap. E27 and E14 are the screw fittings (large and small). B22 and B15 are bayonet. GU10 is the twist-lock spotlight. A bulb with the wrong cap simply will not fit.
The shape. A golf ball, a candle, a globe and a standard GLS bulb are different sizes; a bulb that fits electrically can still be too long for the shade.
Dimming
Not every LED dims. A non-dimmable LED on a dimmer circuit will flicker, buzz, or fail early. The box says so.
Even dimmable LEDs can misbehave on older dimmers designed for the much larger load of incandescent bulbs. If a dimmed LED flickers or will not go low, the dimmer is usually the problem, and a trailing-edge LED-compatible dimmer solves it.
Some warm-dim bulbs also shift colour as they dim, from 2,700K down toward 2,200K, mimicking the way a filament bulb goes amber as it fades. It is a genuinely nice effect in a living room.
Lifespan and what shortens it
LEDs are usually rated at 15,000 to 50,000 hours. At three hours a day, 25,000 hours is over twenty years — and almost no bulb lasts that long in practice.
What actually kills them is heat. The LED itself lasts; the driver electronics in the base cook. So:
- Enclosed fittings run hotter and shorten bulb life considerably. Look for bulbs marked as suitable for enclosed fixtures.
- Downlights in an insulated ceiling are the worst case for heat.
- Cheap bulbs cut cost on the driver first, which is why they fail early rather than dimly.
A quick way to choose
- Check the cap and the shape against the fitting.
- Pick lumens from the old wattage you are replacing.
- Pick 2,700K for living rooms and bedrooms, 3,000–4,000K for kitchens and work areas.
- Look for CRI 90+ where you look at food, faces or clothes.
- If it is on a dimmer, buy dimmable and expect the dimmer may need replacing too.
More household notes are filed under Everyday Essentials.