Any motor "Rated" by the manufacturer at 1500W isn't legal on UK roads, unless it's registered as a motorbike. You really need to read this whole thread:
I think many of you will be aware of what a complex subject electric assist bicycle law is and pitfalls in trying to second guess it. As a result, and with my knowledge of it since joining the trade 70 years ago, I'm often called upon in here to give answers on the subject. But of course I...
www.pedelecs.co.uk
You need to read the whole thread
"(h) pedal cycles with pedal assistance which are equipped with an auxiliary electric motor having a maximum continuous rated power of less than or equal to 250 W, where the output of the motor is cut off when the cyclist stops pedalling and is otherwise progressively reduced and finally cut off before the vehicle speed reaches 25 km/h "
This is the key point pedelecs are only allowed a maximum continuous rating of 250w or less.
It doesn't have a burst limitation which is why bosch motors in prebuilt bikes can peak upwards of 800watts
Also just because a motor is rated for 1500w that ultimately is handled by the controller / display
Motors literally will run at whatever you pump into them.
My hub can easily maintain 15.5mph under 200watts.
Also by reverse logic it's like saying a 250w motor running 2000w through a phaserunner is legal
Because the motor is rated 250w
I even know a guy about to get his 250wlpm on his ebike with the same 1500w hub motor limited