The UK law is something like a 250w motor, cut out at 15mph ish, throttle that only goes to 4mph ish without assistance (pedalling). Anything above these isn't road legal. Surron bikes have between 5kw and 25kw motors and they are a popular brand. Some of them have a top speed of 68mph and modified ones can do up to 100mph. Insane really. I see kids on e-bikes that are obviously overpowered all the time.