But both are legally limited to 250W of power.
that figure is nominal, the guaranteed power that the motor won't overheat.
If you ride on a flat road at 15mph without much headwind, the energy required is about 200W, if you pedal a bit, say you contribute 70W through pedaling, the motor would supply about 130W. There is no way you would need all that 250W on a flat road without headwind.
The 15.5mph speed limit for assistance is on the other hand a permanent legal requirement. That's one key aspect where there is no fudging. That does not mean you cannot exceed it, going downhill for example, but that's where the motor will cut out. With a lightweight bike and fast rolling tyres, you can easily hit 30mph on a slightly downhill road.
E-bikes are allowed to exceed 250W when climbing hills or going against headwind. Typically, they can reach 500W-800W without breaking any law because it is limited to those bits of difficult roads. So heavy people need more help than lightweight people and that's allowed.
The reason that manufacturers choose different high power limit (500W-800W) is because more power needs bigger and heavier motors and batteries. So if you weigh 13st, you only need a about 500W top whereas someone weighing 25st will need something like 800W power on hills. The maximum power is usually indicated by the Amp rating of the controller, 15A for up to 14st, 17A for up to 20st and 20A for 20-25st.
I had considered this but what I think I'm going to do is drive part of the way, park up and cycle the rest of the way. Then reverse it.
