Question: What’s up with the 750 watt motor limit on e-bikes sold in the US? – Mike, Austin, TX
Answer: The ebike law in the US calls for a motor rating of 750 watts. The issue is that motors are labeled with nominal wattage not maximum wattage. Nominal output is the amount of wattage the motor can run at without running into any problems (overheating). To borrow 18th century inventor James Watt’s tactic, think of it in terms of how long you could ride a horse without it having to stop and get a drink of water. Just as a horse will slow down if it gets too hot, so too will a motor overheat if pushed too hard.
Maximum output is the highest number of watts the motor can deliver; however the motor can overheat if run at peak for too long. Think of it as the top speed your horse can run. You may be able to run him at top speed for a while but not over a long period of time. So, a rated 750 watt motor can be run at say 2,000, 3,000 or even 4,000 watts just not for an extended period of time. The motor rating has little to do with how much power the bike has. That’s a function of the speed controller.
An e-bike controller limits the amperage to the motor. Watts (W) = amperage (A) x voltage (V). So if you have a 48V battery the controller can send say 15A for an output of around 720W (48 x 15 = 720). That’s same controller can also send 50A for an output of 2400W (48 x 50 = 2400).
So an e-bike can be certified to 750W but can actually make a lot more power. That’s why e-bike laws are more concerned with top speed than power.