Converting a Bike to Electric: Kits, Cost and the Law

What an e-bike conversion is

An e-bike conversion turns a normal bike into an electric one by adding a motor, a battery and a controller. It lets you keep a bike you already like and add assistance for a fraction of the cost of a new e-bike. The trade-off is that a conversion is rarely as neat or as well integrated as a purpose-built e-bike, and you take on the job of fitting and maintaining it.

Types of conversion kit

There are three common approaches. A hub-motor kit replaces the front or rear wheel with one that has a motor built into the hub. These are the cheapest and easiest to fit, and the most popular. A mid-drive kit, such as the Bafang units, mounts a motor at the cranks and drives the chain, which climbs better but is a more involved fit. A friction-drive kit presses a small roller against the tyre and is the simplest of all, though the least refined.

Whichever you choose, the battery usually mounts on the frame or a rack, and the controller and display wire in along the frame.

What a conversion costs

A conversion is much cheaper than a new e-bike. A kit and battery together typically run from a few hundred pounds up to around eight hundred, depending on the motor type, the battery size and the quality. The battery is the biggest part of the cost, exactly as it is on a purpose-built e-bike, so a bigger range means a dearer kit.

Cyclesite Tip. Start with a good donor bike. A conversion adds weight, so a sound frame, decent brakes and strong wheels matter more once there is a motor and battery on board.

Keeping a conversion road-legal

A converted bike must still meet the UK rules for an electrically assisted pedal cycle to be ridden on the road without registration, tax or a licence. In short, the motor must be no more than 250 watts of continuous power, the assistance must cut off at 15.5mph, and it must only assist while you pedal. A throttle that drives the bike without pedalling takes it outside those rules.

Fitting a kit that is more powerful than the limit, or one with a throttle, means the bike is no longer treated as a normal bicycle in law. Our UK e-bike law guide sets out exactly what is and is not legal before you buy a kit.

Convert or buy a used e-bike

A conversion makes sense if you have a good donor bike you want to keep, you are on a tight budget, or you enjoy the project. A purpose-built used e-bike makes sense if you want a neat, integrated bike with the motor, battery and frame designed to work together, and the reassurance of a known system.

For many buyers a well-kept used e-bike is the better all-round buy, because the motor and battery are matched to the frame and the whole thing is built for the job. Check what used e-bikes cost with the free Cyclesite valuation, or browse used electric bikes to compare against the cost of converting.

Common questions

Is it cheaper to convert a bike to electric or buy an e-bike?

Converting is usually cheaper upfront, from a few hundred pounds for a kit and battery, while a purpose-built e-bike costs more but is neater and better integrated. If you have a good donor bike and a tight budget, converting can make sense. For a tidy, reliable everyday bike, a well-kept used e-bike is often the better buy.

Are e-bike conversion kits legal in the UK?

A converted bike is legal to ride on the road as a normal bicycle only if it meets the rules for an electrically assisted pedal cycle: a motor of no more than 250 watts, assistance that cuts off at 15.5mph, and power only while you pedal. A more powerful motor or a throttle takes it outside those rules.

Which conversion kit is best?

Hub-motor kits are the cheapest and easiest to fit and suit most riders. A mid-drive kit climbs better and drives through the gears but costs more and is a harder fit. Match the kit to your riding, and start with a sound donor bike because a conversion adds weight.