What does a 250 watt bicycle motor mean?

Electric bicycles are rated in terms of the power rating of their motors. But what does it mean to have a “250 watt” motor on your bike?”


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I’ve tried a number of e-bikes over the past couple of years and reviewed a handful of others. Motor sizes have ranged from 250 watts to 500 watts on these bikes.

These wattages don’t ever seem to match reality. My legs can generate about 180 watts during an hour long ride. Amateur racers my size can produce anywhere from 200 watts to well over 300 watts in an hour long effort, while professional elites can pump out 400 watts. On level ground and negligible wind, these wattages translates into anywhere from 18 MPH for me to nearly 30 MPH for the elites.

I’m hand-waving a little here, but you’d expect a 250 watt motor to push you at over 20 MPH, right? In my reviews, I disable the speed governors required by US Federal law and crank the throttle. Without assistance from my legs, 12 MPH on a 250 watt motor as been about typical. It takes a 500 watt motor to push me and the bike to the legally allowed 20 MPH. A cyclist with 500 watt legs, on the other hand, can hit 30 MPH.

I never understood this disconnect between an electric motor’s rated capacity and the actual speed you can get out of it, and I remained skeptical of Specialized’s claim for 28 MPH out of their 250W motor on their Turbo electric bicycle. If you look at my Strava track from Wednesday’s demo ride of the Turbo, however, you’ll see me going well over 20 MPH for much of the ride, and I wasn’t putting much more than 70 or 80 watts of my own effort into that. (Traffic, red lights, and mumerous regroups account for the stop & go nature of that ride in San Francisco). This is a motor that actually delivers 250 watts of power to the wheel.


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When I asked Specialized’s electrical engineer about the lack of performance on other 250W motors, he said, “Motor ratings are such a lie.” He also explained that the nominal rating on a motor doesn’t take into account how much the battery can deliver.

And oh boy, does the battery and motor on the Turbo ever deliver. I’m not really a fan of electric bicycles, but this was one fun rig to ride. It’s as zippy as it looks in spite of the 45 lb weight and beefy rims. The frame is stiffer than any other electric bike I’ve tried (and I’ve tried several), but the 700×45 slicks provide plenty of cushion, while stiff sidewalls on those tires provide the confidence to take turns at speed.


700x45 tire - Specialized Electrak

I hopped a few curbs with this bike. I never managed more than a half inch on bunny hops because the bike is so heavy, and my timing was off on a couple of curb mounts. On most any other bike, hitting a curb square on with the rear wheel at 15 MPH would have resulted in a pinch flat and maybe a dented rim, but the “Electrak” tires handled these hits like a pro.

Headlamp and tail light controls with battery status display are integrated into a single compact dashboard along with traditional cyclocomputer functions of speed and riding distance. The dashboard also can communicate with your ANT+ heart rate monitor.

Specialized is delivering these to dealers in the USA this week and will be available from a limited number of them beginning this weekend.

What about that Federal 20 MPH limit?

U.S. Federal law defines “low speed electric bicycles” that are exempt from motor vehicle equipment regulations as “a two- or three-wheeled vehicle with fully operable pedals and an electric motor of less than 750 watts (1 h.p.), whose maximum speed on a paved level surface, when powered solely by such a motor while ridden by an operator who weighs 170 pounds, is less than 20 mph.


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The Turbo has no hand throttle — it is always in pedal assist mode. You must pedal and apply some torque before the motor provides any additional thrust. Thus, there’s no way for the Turbo ebike to be “powered solely by … a motor.” Specialized tells me their lawyers have assured them that this legal loophole is sufficient to allow them to set the electronic speed governors to 27.9 MPH. I’m no lawyer so I’ll just have to take their word on that.

Buyers should be aware of local and state laws that can apply to electric bicycles. California, for example, has a mandatory helmet law for ebike use. Riders must be at least 16 years old. By default, ebikes are not allowed to be operated on bike paths unless it’s otherwise allowed by local ordinance. That means we may have been breaking state law Wednesday during our fun ebike ride as we terrorized pedestrians and joggers on portions of the San Francisco Bay Trail. Oops.

This bike is fast enough that we easily kept up with the other traffic, at least in San Francisco. I’d personally avoid path riding on this bike at full power.


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Many people gasp at Specialized’s $5900 asking price for the Turbo. To be honest, there are several bills I’d pay before I’d plunk down that kind of change for an e-bike. The Zero XU electric motorcycle has a similar range as the Turbo, but can take you there at highway speeds, and costs only a couple of grand more, most of which can be recovered in Federal and state tax incentives.

Still, there’s a certain practical flexibility to owning a bicycle. Charging a motorcycle battery is challenging for an apartment dweller like me, and there’s still the stupid fun of riding a bicycle that even motorcycle riders miss out on.

8 Comments

  • Andy
    April 26, 2013 - 10:36 am | Permalink

    My stance morphs over time, but currently I think that any vehicle powered over about 10mph should require some kind of licensing or at least registering. I get that people with mobility issues or without the power to climb a hill might be interested in e-bikes, but I don’t think that allowing people to go 20 or 28mph with little of their own effort is really any different from a moped. A 10mph power limit would mean they could still climb any hill (probably faster than even a commuter cyclist on some hills) with little of their own effort. For NH and NY, I had a license, registration, and insurance to drive mopeds that only went 25mph on flat ground – why should a nimbler (aka more dangerous) bike be exempt from that?

    And I think anything powered by more than just my legs (or maybe arms) doesn’t belong on rec trails if it goes faster than walking speed. Wheelchairs make plenty of sense, but cruising along at fast bicycle speeds amongst ipod zombie runners, strollers, leased dogs, etc. is just an awful idea. I’ve heard of some paths with speed limits, but I doubt there’s ever enforcement for that. Most paths I’ve been on were treacherous to share even at 10mph.

  • April 26, 2013 - 7:09 pm | Permalink

    Electric motors are usually rated by input power, not output power. High-power brushless motors have efficiencies over 90%, but low-power ones often have efficiencies under 80%. Full power on an ungeared motor is generally only delivered over a fairly narrow range of speeds—perhaps not the range of speeds you were riding at. At much lower speeds, you are limited by the torque of the motor, and at much higher speeds by the RPM. A good motor controller on a brushless motor can spread the sweet spot out quite a bit, but not indefinitely.

  • April 27, 2013 - 10:07 am | Permalink

    Andy, in the UK the limit for electric vehicle without licence is set at 15mph and that works pretty well. I’d rather manufacturers went for increased range (or lighter weight) rather than faster speeds, then electric bikes would be really useful.

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  • April 28, 2013 - 8:45 am | Permalink

    Couldn’t the extra weight of the motors, batteries and sturdier frame account for the difference in effective speed from similar wattage?

  • April 29, 2013 - 7:21 am | Permalink

    We should really be focusing on calming other urban traffic closer to bike speeds, not speeding up bikes closer to vehicle traffic speeds. I agree that the standard in the UK and much of the rest of the EU of regulating e-bikes at 15 mph or less is more appropriate, as it puts them right in line with standard bike speeds and therefore sets up a lot fewer conflicts between bike lane users. Going faster than 15 mph is still possible using the pedal drive, but traveling at 20 mph+ safely on a bike (and stopping) requires skill that should be learned through experience, and not at the push of a button.

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  • Sarah Joseph
    May 16, 2013 - 11:57 pm | Permalink

    Same sort of ban was implemented on e-bikes in China in early 2002. But now Chinese government has also imposed some speed limits & dedicated e-bike lanes on riding e-bikes on road. I think NYC Government can take reference to Chinese Traffic Laws.

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