Standing in an aisle holding two boxes that look almost identical, one priced twice the other, is a familiar moment. The difference is nearly always printed on the side of the pack in small grey type: a handful of numbers and letters telling you how bright the lamp is, what colour its light will be, whether it will flatter a kitchen worktop, and whether it will survive a steamy bathroom. Learn to read four or five of those figures and you can stop guessing.

Lumens first, watts second

Watts measure the power a lamp draws from the mains, not the light it produces. That distinction mattered less when nearly every bulb was an incandescent, because a 60W filament lamp always gave you roughly the same brightness. LEDs broke that link: a 9W LED can easily outshine an old 60W bulb while using a fraction of the electricity.

So look for lumens (lm) before anything else. As a rough guide to the old wattages most of us grew up with:

  • 450 lm replaces an old 40W bulb
  • 800 lm replaces 60W
  • 1,100 lm covers 75W
  • 1,500–1,600 lm stands in for 100W

“60W equivalent” on a box is shorthand for that output. It is a statement about brightness, not about how much energy the lamp uses.

Divide lumens by watts and you get efficacy, measured in lumens per watt. A decent everyday LED manages roughly 80 to 120 lm/W. Decorative filament-style LEDs often sit nearer 60 or 70, which is one reason a vintage-look bulb can feel disappointingly dim in a pendant. For the same light, a higher figure means a lower bill.

Colour temperature: which white are you buying?

Colour temperature is written in kelvin (K) and describes the warmth or coolness of the white light, not its brightness.

  • 2700K — warm white, close to a traditional filament lamp. The default for living rooms, bedrooms and lamps.
  • 3000K — soft white, slightly crisper. Good for kitchens and hallways.
  • 4000K — cool white. Useful in utility rooms, garages and workspaces; harsh in a bedroom.
  • 6500K — daylight. Very blue, best avoided indoors except in specific task lighting.

A practical tip: stay within a step or two of one temperature throughout a room, or the mismatch becomes obvious. Two lamps with the same kelvin figure from different brands can still look slightly different, so where consistency matters, buy them together.

CRI: why some light makes everything look grey

Colour rendering index runs from 0 to 100 and describes how faithfully a lamp shows colours compared with natural light. Most cheap household LEDs score around 80, which is perfectly serviceable in a corridor.

Where it starts to matter is anywhere you look at things closely: a bathroom mirror, a kitchen worktop, a desk where you match fabrics or paint swatches. Look for CRI 90 or above. The difference is subtle on a specification sheet and obvious in the room — skin looks less sallow, wood keeps its warmth, reds stop looking muddy. Some packaging quotes an “R9” figure too, which measures red rendering specifically; a low R9 is the usual culprit when a lamp makes food look unappetising.

If you only upgrade one lamp in the house, make it the one above the bathroom mirror. It is the one you look into every day.

IP ratings: two digits, read separately

An IP rating tells you what a fitting can withstand, and the two digits mean different things. The first is protection against solid objects, from 0 (none) to 6 (dust-tight). The second is protection against water, from 0 to 9.

  • IP20 — dry indoor use only. Fine for most ceiling lights and table lamps.
  • IP44 — protected against splashes. The common minimum for bathroom lighting.
  • IP65 — dust-tight and resistant to water jets. Suits outbuildings and some exterior fittings.

If there is no IP number, treat the lamp as indoor and dry. In a bathroom, the closer a fitting sits to the bath or shower, the higher the protection needs to be, and the rules on zones are set out in the wiring regulations. Check the current requirements or ask a qualified electrician rather than assuming splash-proof is enough.

The small print worth checking

Cap and fitting

Match the letter and number to your existing lamp: E27 and B22 are the standard large screw and bayonet caps, E14 and B15 the small ones, GU10 and G9 common in spotlights and downlights. Buying the wrong cap is the single most common reason a bulb goes straight back to the shop.

Dimming

“Dimmable” on the box does not guarantee it will dim well with your existing switch. Older leading-edge dimmers and cheap LEDs are a poor pairing, producing flicker, buzz or a narrow range between off and full. If you are fitting several dimmable LEDs, consider having a compatible trailing-edge dimmer installed.

Voltage, life and beam angle

Some spotlights run on 12V through a transformer, others on 230V mains; they are not interchangeable. Lifespan is quoted in hours, and realistically you should look for the “L70” figure, which is the point at which output has dropped to 70 per cent — bulbs do not usually fail outright, they fade. Beam angle matters for spots: a narrow beam punches down, a wide one washes a wall.

The energy label, decoded

Since labels were rescaled, lamps are graded from A to G rather than A+++ to E. Most ordinary LEDs land at E or F, which looks alarming and means nothing bad has happened — the scale simply moved. The label also shows lumens in a box, the wattage, and energy use in kWh per 1,000 hours, which is the most useful figure for comparing running costs between two similar lamps.

Buying with confidence

Next time you are in the aisle, work through it in order:

  1. Check the cap and voltage match what you are replacing.
  2. Choose lumens for the brightness you actually want.
  3. Pick a kelvin figure that suits the room.
  4. Pay a little more for CRI 90+ in kitchens and bathrooms.
  5. Confirm the IP rating if the fitting is anywhere damp, and check dimmer compatibility before you commit.

Get those five things right and the lamp will do exactly what you expect — which, for something you might not think about again for a decade, is the whole point.

Photo: qimono / Pixabay