An e-bikes power is measured and expressed in ‘Newton metres’ (Nm) – a measure of Torque, being the amount of assistance you can call on via your system’s setting and motor. It is the motor’s pulling/accelerating/climbing power.
Legal 250w e-bike Hub and Mid drive motors are set at the point of manufacture to produce varying levels of torque. Hub drives produce around 30 – 60Nm, Mid drives 40Nm up to as much as 100Nm or more, similar to a Ford Fiesta!
Basically you need a powerful enough motor to meet your particular needs and make for safe and relaxed riding for your area/usage.
In our experience in hilly Sheffield, a rider using an e-bike for relatively light duties like commuting and is not themselves very heavy will manage most gradients with a bit of effort using a mid drive or hub drive system delivering a minimum of 40 Nm of torque (assistance).
However most riders, particularly those using the bike’s potential to carry loads in hilly areas, really will need a mid drive with a minimum of around 50 Nm. If you plan to really load up, tow a trailer, or would just prefer a really relaxed ride where your journey involves proper Sheffield hills, 60Nm and upwards mid drive motors will serve you well.
Currently that would mean choosing for example no lower spec than a Shimano 6100 series motor or the equivalent ‘Performance Line’ Bosch motor.
Note: Although often assumed, in real life there is very little to be gained from models that ‘regenerate’ electric as you freewheel, all of which are direct Hub drive anyway.