I’ve fitted smart bulbs into three different rented flats and my own 1930s semi over the past few years, and the same problem comes up every single time: the box arrives, the app installs fine, and then the bulb just won’t twist or screw into the fitting I already have. It’s rarely a wiring issue. It’s almost always a mismatch between B22 and E27, the two fittings that dominate British homes.
This isn’t a global lighting standard question. It’s a very British one, because the UK is one of the few markets where both bayonet and screw fittings are common in the same housing stock, sometimes in the same room. Get the fitting wrong and you’ve either bought a bulb you can’t use or you’re reaching for an adaptor that, in some cases, shouldn’t be trusted at all.
What B22 and E27 Actually Mean
B22, also called a bayonet cap or BC fitting, has two small metal pins on either side of the base. You push the bulb up into the holder and give it a quarter turn clockwise until the pins lock under two spring-loaded contacts. It’s a mechanical connection, not a threaded one, which is why bayonet bulbs can feel slightly loose or wobbly compared with a screw fitting, especially once a holder has had a few years of bulb changes.
E27, the Edison screw cap, is a threaded metal base measuring 27mm across that you turn clockwise to wind into a matching socket, the same way you’d tighten a jar lid. It’s named after Thomas Edison, whose early incandescent bulbs used this design, and it’s the fitting used across most of continental Europe and much of Asia today.
Neither fitting is inherently “better” from an electrical standpoint. B22 became the standard in the UK and much of the former British Empire and Commonwealth, including Australia and large parts of India, largely through historical convention rather than any technical advantage. E27 spread globally on the back of Edison’s own manufacturing dominance. The UK ended up with a domestic market shaped by British bayonet convention while importing enormous volumes of screw-fitting lamps and appliances from Europe, which is why both sit side by side in most British homes rather than one having fully replaced the other.
It’s worth flagging a third fitting here too, because it trips people up in the same conversation: GU10. That’s the twist-lock spotlight base you’ll find in most kitchen downlights and bathroom spots fitted in the last twenty years. It isn’t interchangeable with either B22 or E27 and isn’t really part of this comparison, but if you’re auditing your whole house before buying smart bulbs, don’t assume every socket is one of the two fittings covered here.
How to Check Which Fitting You Actually Have
The quickest way is to remove your existing bulb and look at the base. Two metal pins sticking out sideways means B22. A threaded metal collar means E27. If you don’t want to remove a working bulb, most manufacturers print the fitting code somewhere near the base, though it can be small and worn on older bulbs.
In my experience, older housing stock, particularly Victorian and Edwardian terraces with original ceiling roses, tends to run B22 throughout the main pendant fittings, even where later renovations have swapped in E27 table and floor lamps bought from IKEA, Dunelm or Habitat. Newer builds and recently rewired properties are more mixed. There’s no official ONS or government breakdown of how many UK homes use each fitting, so treat any precise percentage you see elsewhere with some scepticism. What I can say with confidence, having crawled around enough loft hatches and ceiling roses, is that checking room by room before you order anything will save you a returns hassle later.
Real Smart Bulb Options You Can Actually Buy Right Now
The good news is that every major smart bulb brand sells both fittings, usually at identical prices, so the choice never limits your features or your budget. Here’s what’s currently available from mainstream UK retailers:
| Brand & Model | Fitting | Approx. Price | Hub Required? |
|---|---|---|---|
| Philips Hue White (twin pack) | B22 or E27 | Around £30–£35 for two | No for basic Bluetooth control; yes (Hue Bridge, ~£60) for full app scenes and voice routines beyond 10m range |
| TP-Link Tapo L510B/L510E (white, dimmable) | B22 or E27 | £7.99 at Argos | No, connects directly to home Wi-Fi |
| TP-Link Tapo L530B/L530E (colour) | B22 or E27 | £7.99 at Argos | No |
| IKEA TRÅDFRI | B22 or E27 | £8 each | Battery remote works standalone; DIRIGERA hub or TRÅDFRI gateway needed for app control and Zigbee integration |
The TP-Link Tapo range is the one I’d point most people toward if this is their first smart bulb purchase, purely because there’s no hub to buy on top and the price barely moves whether you pick white or colour-changing. The Tapo L510B I’ve been running in a bayonet hallway fitting for over a year now has been reliable, though its 806 lumen output and F energy rating (Argos lists it under the post-2021 rescaled A–G label) means it’s not the most efficient bulb on the market. It’s fine for a hallway; I wouldn’t put it in a room where I cared about running costs over several years.
Philips Hue remains the more polished option if you’re building out a full smart lighting system with the Hue Bridge, motion sensors and dimmer switches, but you’re paying a noticeable premium for that ecosystem. IKEA’s TRÅDFRI sits in between: genuinely good value at £8 a bulb, but you’ll want the DIRIGERA hub if you want proper scheduling and voice control rather than just a physical remote.
Does the Fitting Itself Change Anything About Performance?
Not really, and this is the point that gets lost in a lot of buying guides. The electronics inside a B22 smart bulb and its E27 sibling from the same product line are identical: same LED chips, same Wi-Fi or Zigbee radio, same app, same brightness and colour temperature options. The cap is just the physical connector. Manufacturers make both versions from the same production line and swap the base assembly at the end.
Where it does matter is safety around adaptors. If you’ve got a B22 fitting but want an E27 bulb (or the reverse), it’s tempting to buy a cheap cap adaptor rather than the correct bulb. Be careful here. The UK’s Office for Product Safety and Standards has recalled several B22-to-E27 adaptors after finding exposed live parts that posed an electric shock risk. Given that a correctly-fitted bulb usually costs the same either way, it’s rarely worth the risk of a sub-£3 adaptor from an unverified seller. If you genuinely need to bridge fittings long-term, it’s safer to replace the lampholder itself, ideally with a qualified electrician if it’s a ceiling rose rather than a plug-in lamp.
One Caveat Nobody Mentions: Old Dimmer Switches
Fitting is only half the compatibility question. If the room has an existing physical dimmer switch on the wall, whether the bulb is B22 or E27 is almost irrelevant next to whether that dimmer plays nicely with a smart LED. Most Tapo and Hue bulbs explicitly say not to use them with a wired wall dimmer, because the bulb’s own app-based dimming conflicts with the switch cutting the mains voltage, which can cause flickering or premature failure. IKEA is upfront about this too on the TRÅDFRI product pages.
The fix is straightforward but easy to overlook: replace the wall dimmer with a standard on/off switch and let the bulb handle dimming through its app or Zigbee remote instead. I made this mistake in my own living room, chasing a flickering Hue bulb for a week before realising the fifteen-year-old rotary dimmer was the culprit, not the bulb or the fitting. It’s a five-minute job for anyone comfortable with switching off the consumer unit first, or a short call-out for an electrician if you’d rather not touch the wiring yourself.
Zigbee, Wi-Fi and Bluetooth: A Separate Decision From the Cap
It’s easy to conflate “which fitting” with “which smart home protocol”, but they’re unrelated choices. Tapo bulbs connect straight to your home Wi-Fi, no hub, in either B22 or E27. Hue and IKEA use Zigbee, a lower-power mesh protocol that talks to a dedicated bridge rather than your router directly, again regardless of which cap you buy. If your Wi-Fi router already struggles with device count, a Zigbee-based system might suit you better long-term, but that’s a decision about ecosystem, not about bayonet versus screw.
What I’d Actually Recommend
If you’re renting, check the fitting on the bulb you’re removing before you order anything; most landlords won’t thank you for swapping ceiling roses, and returns on the wrong fitting are avoidable admin nobody needs. If you’re kitting out a whole house, don’t assume consistency. Walk each room, note B22 or E27 next to each fixture, and buy accordingly rather than guessing from the newest lamp you happen to remember.
For most first-time buyers, I’d start with a single Tapo L510B or L510E at £7.99, live with it for a fortnight, and only then commit to a full Hue or TRÅDFRI setup if you decide you want scenes, sensors and the wider ecosystem. The fitting itself won’t make that decision for you. It just needs to be right before the bulb arrives.