Make a short specification note
Record the cap code, bulb shape and dimensions. A bayonet cap and a screw cap are not interchangeable, and similar-looking screw fittings can have different diameters. Check the fitting's instructions and rating rather than relying only on a photo. A new bulb may share the right cap but be too long or wide for the enclosure.
Confirm whether the fitting has a dimmer and whether the bulb and dimmer are compatible. Not every LED bulb is dimmable, and a dimmable label alone does not establish that it will work with every existing control. If the combination is unclear, ask the manufacturer or an electrician rather than trying improvised electrical modifications.
Compare light, not just power
Lumens describe light output, while watts describe power consumption. Use lumens when comparing the intended brightness of replacement bulbs, instead of looking for the wattage of an older design. The distribution of light and the shade will still affect how the room looks, so a matching number does not guarantee an identical effect.
Check the colour temperature as well as a name such as warm white. Names can cover a range, whereas the Kelvin figure gives a clearer comparison. If several bulbs are visible together, choose a consistent specification unless you deliberately want different lighting effects. Buy one suitable example first if you are uncertain about the appearance.
Choose the right quantity
Count the fittings that actually use this specification. A cheaper pack of six may leave five unsuitable spares if most lamps have a different cap. Check the price for the selected option, because listings sometimes combine single bulbs, packs and different brightness levels on one page.
Keep the packaging information until you know the bulb fits and works as intended. For integrated light fittings, the light source may not be a separately replaceable bulb at all. Establish what can be replaced before ordering, and compare the correct component rather than trying to make an attractive bulb offer fit the wrong fixture.
