Torque vs power: what Nm and W mean on an e-bike
Two numbers get quoted for every e-bike motor, and they are routinely confused with each other. Watts (W) measure power: how much work the motor can do per second. Newton-metres (Nm) measure torque: how much turning force it can apply.
For how an e-bike feels on a hill, torque is the more useful number. For how fast it can go and how the law treats it, power is. Here is what each one actually tells you.
Power: watts
A watt is one joule per second. On an e-bike, the rated figure is continuous power: what the motor can sustain without overheating, not what it can peak at.
Continuous power determines two things:
- Top speed under assist, which in the UK is capped at 15.5 mph (25 km/h) by law, regardless of the motor's rating.
- The legal category of the machine. Up to 250W continuous, with assistance cutting at 15.5 mph, it is an Electrically Assisted Pedal Cycle. Above that it is a motor vehicle.
What continuous power does not tell you is how hard the bike pulls away from a standstill or how it copes with a 12% gradient. That is torque.
Be sceptical of "peak power" figures. A motor rated 250W continuous might peak at 500-600W for a few seconds on a climb. That is normal and legal, because the regulation is written around the continuous rating. A seller quoting "750W peak" on a 250W motor is describing exactly that, not a 750W motor.
Torque: newton-metres
Torque is force multiplied by distance from the pivot. On a bike it is the turning effort at the crank or the wheel.
A rider pedalling hard might produce 60-80 Nm at the cranks in a sprint, and 20-30 Nm cruising. An e-bike motor typically adds 30-85 Nm, depending on the system and how it is geared.
Torque is what you feel:
- Pulling away from traffic lights. High torque gets you moving.
- Climbing. Torque against gravity, multiplied by your gearing.
- Carrying a load. Torque is what compensates for the extra mass.
- Riding into wind. Same story: resistance goes up, torque is what answers it.
This is why a 250W motor with 80 Nm can feel dramatically stronger up a hill than a 250W motor with 40 Nm, even though the law and the top speed are identical.
Why the two numbers do not track each other
Power is torque multiplied by rotational speed:
Power (W) ≈ Torque (Nm) × Angular velocity (rad/s)
A motor can produce high torque at low speed (a hub motor at walking pace, straining) or low torque at high speed (a motor spinning fast on the flat). The same power output can be delivered either way.
In practice:
- Hub motors tend to be high-speed, low-torque units geared down inside the hub. They produce their torque at wheel speed, not crank speed, so it cannot be multiplied by your gears.
- Mid-drive motors produce torque at the cranks, which then goes through your cassette. A 60 Nm mid-drive in a low gear can deliver far more torque at the wheel than a 60 Nm hub motor.
That is the single most important thing in this article: motor torque is not wheel torque. The drivetrain multiplies it, and only a mid-drive gets that multiplication.
What the numbers mean in practice
These are the ranges you will see on UK-legal e-bikes (250W continuous), and what they feel like:
| Motor torque | What to expect |
|---|---|
| 30-40 Nm | Light assistance. Fine on flat ground; you will work on hills. |
| 40-55 Nm | The common commuter range. Copes with rolling terrain and a moderate load. |
| 55-70 Nm | Noticeably strong. Handles steep hills and heavy riders without strain. |
| 70-85 Nm | Cargo-bike territory. Pulls a loaded bike up a hill; also wears drivetrains fast. |
Above about 85 Nm, you are usually looking at motors that are not 250W, and therefore not road-legal in the UK.
The trap: comparing Nm between hub and mid-drive
Because a mid-drive's torque goes through the gears, a 50 Nm mid-drive will out-climb a 60 Nm hub motor on a steep gradient. Comparing the raw figures across the two designs is meaningless, and sellers know it.
The fair comparison is at the wheel, and almost nobody publishes it. So use the numbers as a rough guide within a design type, and judge across types by test-riding or by reading what the bike is designed for.
What actually predicts how a bike feels
In order of practical impact:
- Motor position (mid-drive or hub) — see our comparison of hub and mid-drive motors.
- Torque at the crank, for mid-drives.
- Total system weight — bike plus rider plus load. A 30 kg bike with a 95 kg rider is a different proposition from a 22 kg bike with a 70 kg rider, and no motor figure hides that.
- Battery capacity in watt-hours, which decides how long the assistance lasts, not how strong it is.
- The assist mode you ride in, which can halve or double the range.
A word on the legal numbers
If you are checking whether a bike is road-legal in the UK, the two figures that matter are:
- Continuous motor power: 250W maximum.
- Assistance must cut out at 15.5 mph.
Torque is not regulated. A 250W motor with 85 Nm is perfectly legal. A 500W motor with 40 Nm is not, whatever it feels like.
Our UK e-bike legality checker takes the power and speed figures from a listing and tells you which category the machine falls into in one step, and our guide to UK e-bike law explains the reasoning behind it.