Ed Benjamin from the Light Electric Vehicle Association recently detailed some considerations when thinking about buying an ebike - specifically, motor size - 250, 500, or 750 watts. Since legal definitions of ebikes are partially based on wattage, it's good to understand the difference. Here's a summary of his considerations:
1. What is actually legal where you live? In Oregon and Washington, the motor limit is 1000 watts, but there are other details in the definition as well.
2. How is that power measured? A motor may different power ratings based on where its measured.
3. Are you measuring peak power, or continuous?
4. How much power will your battery realistically drive - range/weight/cost are all factors here.
In general, Ed says that 250 watts are good for pedal assisted systems - as the rider has to be pedaling to for the motor to engage, they'll always be helping move the bike. 350-500 watt motors are better, Ed says, in throttle-based applications, and that much bigger than 750 watts really starts to require impractical battery arrangements.
Thanks for the tips, Ed! One of Ed's best points, I think, is that ebikes are bicycles, and "a bicycle is a very privileged vehicle". Ed was also recently interviewed on NPR's Science Friday, which I transcribed/summarized here.