How much Power do I really need?
As a general rule, a cyclist can produce 100 watts for a reasonable length of time. To be really useful, a motor needs to produce another 100 Watts on a continuous basis, with a peak power of at least 300 watts. Just to confuse things, our measurements are of power consumption - losses in the motor and drive system mean that the power output to the wheel can be much lower. If you expect the motor to do most of the work, especially in a hilly area, you'll want a peak consumption of 600 watts or more. On the other hand, if you prefer gentle assistance, a peak of 200 watts may be enough.