How Electric Bikes Work

An electric bike is a normal bicycle with three extra parts bolted on: a motor, a battery and a sensor that links the two. You still pedal exactly as you would on any bike. The difference is that when you push the pedals, the sensor tells the motor to feed in power, so every turn of the cranks goes further with less effort. The motor never takes over completely on a UK-legal bike, it just amplifies what your legs are already doing.

That single idea, assistance rather than replacement, is what separates a road-legal electric bike from a moped or an electric motorbike. Below we break down each part, explain how pedal assist actually decides how much help to give, and cover the throttle and UK law questions that confuse most first-time buyers.

The three parts that make an e-bike work

Every pedal-assist electric bike comes down to three components working together.

The motor

The motor is what adds the push. On most UK e-bikes it sits in the rear wheel hub (a hub motor) or in the centre of the bike by the pedals (a mid-drive motor). A hub motor is cheaper and quieter; a mid-drive feeds power through your gears, which makes it stronger on hills and gives a more natural ride. By law a UK electric bike’s motor is rated at no more than 250W of continuous power. We cover the trade-offs in detail in our electric bike motors explained guide.

The battery

The battery stores the energy the motor uses. It is almost always a lithium-ion pack, either bolted into the frame, integrated into the down tube, or removable so you can carry it indoors to charge. Battery size is measured in watt-hours (Wh), and a bigger number means more range. A typical UK e-bike battery holds 250 to 700Wh, enough for a claimed 30 to 65 miles. You recharge it from a normal mains socket in around 3 to 6 hours. Our battery guide explains capacity, lifespan and care.

The sensor

The sensor is the quiet third part that most buyers overlook, yet it shapes how the bike feels more than anything else. It detects that you are pedalling and tells the controller how much power to release from the battery to the motor. There are two types, and the difference is huge.

How pedal assist works: torque vs cadence sensors

Pedal assist, sometimes shown as PAS on the display, is the system that decides how much help you get. You pick an assist level (often Eco, Tour, Sport, Turbo or simply 1 to 5) and the bike scales the motor’s power to match. But how it reads your pedalling depends on which sensor it uses.

A cadence sensor only checks whether the pedals are turning. Once it detects rotation, it switches the motor on at the preset power for your chosen level, like a switch. It is cheaper and works fine, but the power can arrive in an on-or-off surge and there is often a slight delay before it kicks in. Most budget e-bikes use a cadence sensor.

A torque sensor is cleverer. It uses a strain gauge to measure how hard you are actually pushing on the pedals, many times a second, and feeds in power in proportion. Push gently and you get a gentle boost; push hard up a hill and the motor responds instantly with more. This makes a torque-sensor bike feel like a normal bike with strong legs, which is why riders consistently prefer it. The catch is cost: torque sensors are more advanced and usually only appear on pricier models.

Throttle vs pedal assist: what is the difference?

Pedal assist needs you to be pedalling for the motor to help. A throttle lets the motor drive the bike without pedalling, like a twist grip or thumb lever on a scooter. Some e-bikes offer both: pedal assist for normal riding plus a throttle for pulling away from a standstill.

This is where UK law gets specific. On a road-legal electric bike (an EAPC), a throttle is only allowed to power the bike up to walking pace, around 3.7mph, to help you get moving. Beyond that, you must pedal for the motor to keep helping. The exception is a small number of type-approved EAPCs, which are permitted to use a throttle up to 15.5mph without pedalling. Unless a bike is sold as type-approved, treat its throttle as a walking-pace start aid only.

If you want the best of both, look for an EAPC that clearly states it pairs a 250W pedal-assist motor with a legal throttle.

Browse e-bikes with throttle and pedal assist on Amazon

What happens at 15.5mph?

On a UK-legal e-bike the motor stops adding power once you reach 15.5mph (25km/h). The bike does not brake or hold you back, you simply carry on under your own legs as you would on any bicycle, and the assistance returns the moment you drop below 15.5mph again. This speed cap is one of the defining rules of an EAPC, alongside the 250W power limit. Our full UK electric bike law guide covers the detail.

The UK rules in plain English

A legal UK electric bike, an Electrically Assisted Pedal Cycle (EAPC), must meet a short list of rules:

• The motor must be rated at no more than 250W of continuous power.
• Motor assistance must cut off at 15.5mph.
• It must have working pedals that can move the bike.
• The rider must be 14 or over.

Meet those and the bike is treated exactly like an ordinary bicycle: no licence, no road tax, no insurance and no registration required, though theft insurance is well worth considering. You can ride it anywhere a normal bike is allowed, including cycle lanes.

Do electric bikes recharge themselves?

A common myth is that e-bikes top up their own batteries as you pedal or brake. In practice almost none do in any useful way. A handful of systems offer regenerative braking, but it recovers very little energy while adding weight and drag, so most manufacturers leave it out. For nearly every UK e-bike, you charge the battery from a mains socket with the supplied charger.

Why is my pedal assist not working?

If the motor stops helping, the cause is usually simple. Run through these checks before booking a repair:

• Battery: make sure it is charged and clicked firmly into place. A loose or flat battery is the most common cause.
• Assist level: check the display is not set to level zero. It is easy to knock it down by accident.
• Sensor and magnet: the pedal assist sensor relies on a small magnet near the cranks. If it has shifted or the cable has worked loose, assistance stops.
• Connectors: check the motor and display cables are seated, especially after transporting the bike.

If none of that fixes it, a shop can read the error code on the display and diagnose the controller or motor. Resist the temptation to bypass the sensor, as a poorly done bypass can make the bike unsafe and is likely to void the warranty.

So, how do electric bikes work? The short version

An electric bike adds a motor, a battery and a sensor to a normal bike. You pedal, the sensor measures your effort, and the motor feeds in proportional power up to 15.5mph, when the assistance bows out and you ride on under your own steam. A torque sensor makes that help feel natural; a cadence sensor makes it cheaper. Keep it at 250W with working pedals and you have a road-legal EAPC that needs no licence, tax or insurance.

If you are weighing up whether one is right for you, our electric bikes UK overview and our range explained guide are the natural next reads.