How Electric Bikes Work

An electric bike works by adding three extra parts to a normal bicycle: a motor, a battery and a sensor that links the two. You still pedal exactly as you would on any bike. The sensor tells the motor to feed in power when you push the pedals. Every turn of the cranks then 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.

Assistance rather than replacement is the single idea that separates a road-legal electric bike from a moped or an electric motorbike. Below we break down each part. We explain how pedal assist decides how much help to give. We also 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. This setup is called a hub motor. Some sit in the centre of the bike by the pedals instead. This setup is called a mid-drive motor. A hub motor is cheaper and quieter. A mid-drive feeds power through your gears. That 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. Packs are 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). A bigger number means more range. A typical UK e-bike battery holds 250 to 700Wh. That is enough for a claimed 30 to 65 miles. You recharge it from a normal mains socket in around 3 to 6 hours. Battery capacity, lifespan and care all shape how a bike performs over time.

The sensor

The sensor is the quiet third part that most buyers overlook. It shapes how the bike feels more than anything else though. It detects that you are pedalling. It tells the controller how much power to release from the battery to the motor. Two types exist. The difference between them is huge.

How pedal assist works: torque vs cadence sensors

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

A cadence sensor only checks whether the pedals are turning. It switches the motor on at the preset power for your chosen level once it detects rotation. This works like an on-off switch. It is cheaper and works fine. The power can arrive in an on-or-off surge though. There is often a slight delay before it kicks in too. 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. It does this many times a second. It then feeds in power in proportion to your effort. 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. That 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. Think of 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.

UK law gets specific here. A road-legal electric bike is called an EAPC. Its throttle is only allowed to power the bike up to walking pace (around 3.7mph) to help you get moving. You must pedal beyond that point for the motor to keep helping. A small number of type-approved EAPCs are the exception. These are permitted to use a throttle up to 15.5mph without pedalling. Treat a bike’s throttle as a walking-pace start aid only unless it is sold as type-approved.

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

Browse e-bikes with throttle and pedal assist on Amazon

What happens at 15.5mph?

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

Our full UK electric bike law guide covers the detail.

The UK rules in plain English

A legal UK electric bike is called an Electrically Assisted Pedal Cycle (EAPC). It 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. Theft insurance is still well worth considering though. You can ride it anywhere a normal bike is allowed. This includes cycle lanes.

Do electric bikes recharge themselves?

No. Almost none do in any useful way. A common myth claims e-bikes top up their own batteries as you pedal or brake. A handful of systems offer regenerative braking. It recovers very little energy though. It also adds weight and drag. Most manufacturers leave it out for that reason. You charge the battery from a mains socket with the supplied charger on nearly every UK e-bike.

Why is my pedal assist not working?

The cause is usually simple when the motor stops helping. 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. Assistance stops if it has shifted or the cable has worked loose.
• Connectors: check the motor and display cables are seated. This matters especially after transporting the bike.

A shop can read the error code on the display and diagnose the controller or motor if none of that fixes it. Resist the temptation to bypass the sensor. A poorly done bypass can make the bike unsafe. It is also 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. The motor feeds in proportional power up to 15.5mph. The assistance bows out beyond that point. You ride on under your own steam from there. 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.

Our electric bikes UK overview is the natural next read if you are weighing up whether one is right for you. Our range explained guide is worth a look afterwards too.