The 750 W kit everyone recommends, and the law nobody mentions

Everyone buys the same motor

If you have spent an evening reading about converting a bike, you have met the Bafang BBS02B. It is the mid-drive that sits where your bottom bracket used to be, it fits the 68 mm shell that most older bikes have, and it turns a twenty year old hardtail into something that climbs a hill without you thinking about it. The kits are everywhere and they are cheap by the standards of a complete e-bike.

The version that gets recommended, in almost every video and almost every forum thread, is the 48 V 750 W one. That is not an accident. It is the version that feels good, and it is the version that is legal to sell and ride in the United States, where most of the videos are made.

I live in a town where the nearest bike shop that would touch an electric conversion is fifty minutes away, so I read a lot before I buy anything. This is the thing that took me longest to get straight, and it is not a technical problem at all.

What the kit actually is

The published figures for the BBS02B, which are not in dispute.

Why 250 is the number that matters here

A bicycle stops being a bicycle, in European law, at a specific point. The exemption that lets an electric bike be treated as a pedal cycle covers cycles with pedal assistance and an auxiliary electric motor having a maximum continuous rated power of less than or equal to 250 W, where the output is cut off when the cyclist stops pedalling and is progressively reduced and finally cut off before the vehicle reaches 25 km/h.

Read that as three separate conditions, because that is how it works. 250 watts. Cut off by 25 km/h. And the motor stops when you stop pedalling. Fail any one of them and the machine has not merely broken a limit. It has fallen out of the exemption altogether, which means it is a vehicle that needs type approval, registration and insurance, and you need a licence for it.

A 750 W kit fails the first condition by a factor of three. It does not matter that you never use the power, and it does not matter that the display is set to cut at 25. The rating is the rating.

So what do you actually do

Three honest options, and I am not going to pretend the first one is exciting.

Buy the 250 W version. Bafang and everyone else make one, it is legal everywhere in the EU and the UK without any paperwork, and on a mid-drive it is less feeble than the number suggests because the motor gets to use your gears. On a hill in a low gear, 250 W through a cassette does more than 250 W in a hub.

Buy the 750 W version and register it. It becomes a moped in the eyes of the law. Type approval, plate, insurance, helmet rules, and it is no longer allowed on the paths where you probably wanted to ride it. People do this. It is a real choice, it is just a different vehicle.

Or buy it and ride it on private land. Also a real answer, and the one the videos are implicitly describing without saying so.

What I would push back on is the fourth option, which is buying the 750 W kit and assuming that because it arrived without a fuss, it is fine. Customs clearing a parcel is not a legal opinion about what you are allowed to ride to work.