Buying an LED bulb online should take two minutes. You check the shape, add it to the basket and get on with your day — until the parcel arrives and the cap doesn't fit, the dimmer hums, or the "warm white" turns out to look like the inside of a fridge. The good news is that nearly all of these disappointments come from a handful of avoidable mistakes. Here is what to check before you pay.
Fit first: caps, pins and voltage
The cap is the part that connects the lamp to the fitting, and it is the most common reason for returns. Bayonet caps (B22) push in and twist; Edison screws (E27) turn. Smaller versions exist for candle and decorative lamps — E14 screw and B15 bayonet — along with G9 for some ceiling fittings. Then there are the pin-based types: GU10, which twists into mains-powered downlights, and GU5.3, often sold as MR16, which usually runs at 12 volts from a transformer.
Two traps catch people out. The first is the difference between small screw and small bayonet, which look almost identical in a listing photo and are not interchangeable. The second is voltage. A GU10 and a GU5.3 appear similar, but a 12V lamp will not run in a mains fitting, and a mains lamp should never go into a 12V circuit.
Take the old bulb out and look at it properly before ordering. Count the pins. Note whether the base is smooth or threaded. If you are unsure, photograph the bulb next to a ruler and compare it with the dimensions in the listing. Check the physical size of the replacement too: many LED versions are slightly wider or longer than the halogen lamps they replace, which matters inside a tight shade or a shallow recessed downlight.
Dimming: the mistake that shows up later
An ordinary LED on a dimmer circuit will often flicker, buzz, or fail early. Dimming puts stress on a driver that was never built for it. If the switch on the wall is a dimmer, you need a lamp marked as dimmable — and even then, not every dimmable lamp works with every dimmer.
Older dimmers were designed for the resistive load of a filament bulb. LEDs draw very little current, which can fall below the dimmer's minimum load, causing flicker at low settings or a lamp that refuses to switch on at all. Trailing-edge dimmers generally behave better with LEDs than older leading-edge types, and some circuits need one conventional bulb left in place to keep things stable.
The practical step is simple. Dig out the dimmer manufacturer's compatibility list if you can, then buy a single lamp and test it at full brightness, at minimum, and at several points in between. Once it behaves properly, order the rest of the set.
"Warm white" doesn't always mean warm
Colour descriptions on marketplace listings are loose. Warm white is usually around 2700K, though some sellers use it for 3000K. Cool white sits near 4000K. Daylight can mean anything from 5000K to 6500K, and at the top of that range it looks distinctly blue — fine above a workbench or in a utility room, harsh in a living room.
The kelvin figure is the number to trust, not the name on the box. Ignore the product photo as well: cameras and screens shift both colour and brightness, so two listings showing the same cosy glow can arrive looking nothing alike.
It is also worth checking the CRI, or colour rendering index. A CRI of 80 is normal and perfectly acceptable on a landing or in a hallway. In a room where you read, cook, or look at faces, 90 or above makes colours look more natural and skin tones less grey.
Finally, compare lumens rather than wattage claims. "60W equivalent" is vague. A genuine replacement for an old 60W incandescent gives roughly 800 lumens. If a tiny lamp promises the output of a 100W bulb, be sceptical.
Cheap bulbs that fail early
LED lamps rarely fail because the LED itself wears out. They fail at the driver, the small circuit that converts mains power. In a cheap lamp that driver runs hot and its capacitors dry out, sometimes within months. A well-made lamp moves heat away through an aluminium body or a proper heatsink; a cheap one traps it inside a plastic shell.
Warning signs in a listing:
- no brand name, or a name you cannot trace anywhere else
- no UK returns address, or warranty terms that stay vague
- claims that seem too good for the size and the price
- "super bright" with no lumen figure, or a colour temperature left blank
CE or UKCA marking should be present, though it is not a guarantee of quality on its own. If you need six lamps for a room, buy one first and live with it for a week. Keep the order confirmation and the packaging until you are satisfied.
Specs that are easy to overlook
Beyond the cap and the colour, a few details decide whether a lamp suits the space:
- IP rating for bathrooms, kitchens and outdoor fittings — a standard lamp in a steamy zone will not last
- Ambient temperature range for garages, sheds and unheated rooms, where some lamps start slowly or not at all
- Beam angle — a narrow spot and a wide flood behave very differently over a worktop
- Warm-up time, as some budget lamps take a moment to reach full brightness
- Switching behaviour with sensors, timers and smart switches, which some drivers dislike
Before you click
Run through this at the checkout and most problems disappear before they reach your doormat:
- Remove the old lamp and identify the cap by eye, not by memory.
- Write down cap type, voltage, kelvin, lumens and whether dimming is needed.
- Measure the space, including shade clearance and recess depth.
- Check your dimmer's compatibility if one is fitted.
- Buy one lamp, test it, then order the rest.
- Save the order details and keep the packaging until you are confident.
None of this takes long. Two minutes with a tape measure and a torch will spare you a return, an awkward message to a marketplace seller and a drawer full of lamps that almost fit. Buy fewer, better bulbs — and buy them once.
Photo: Mediamodifier / Pixabay
