Is 500 watts enough for an ebike?

QUESTION: Is 500 watts enough for an ebike? I’m in the market for one and I want sufficient power. —Jamie J.

RBR’S STAN PURDUM REPLIES: Generally, yes. In fact, some ebike manufacturers speak of 500-watt motors as the best “all-purpose” size for ebikes.

But for road ebikes, commonly available motor sizes range from 250 to 750 watts, and several factors can help you determine which size is best for you. And in some cases, particularly for riders who really are looking for just a little pedal assist rather than a motor scooter in ebike disguise, the “less is more” principle may apply.

First, it’s useful to know that by law in most states, all legal electric bicycles must fit into one of three classes, defined as follows:

• Class 1: ebikes that are pedal-assist only, with no throttle (a device that operates the motor without the rider pedaling), and have a maximum assisted speed of 20 mph. That is, the motor ceases to provide assistance when the e-bike reaches 20 mph. You can still pedal faster if you are able, but without any motor assist. 
• Class 2: ebikes that also have a maximum speed of 20 mph but are throttle- and pedal-assisted. The throttle does not have to be employed, but both throttle-assistance and/or pedal assistance cease when the e-bike reaches 20 mph. With the throttle, class 2 ebikes can be driven without pedaling, but do not have to be.
• Class 3: ebikes that are pedal-assist only, with no throttle, and a maximum assisted speed of 28 mph. The motor ceases to provide assistance when the e-bike reaches 28 mph.

All classes limit the motor’s power to 1 horsepower (750 watts), which is more than enough to propel most bikes to their legal speed limit while you pedal. Bikes with more than 750-watt motors are legally classed as electric mopeds or even electric motorcycles and must be licensed and insured, and the rider must have the appropriate driving license. Electric bicycles with motors of 750 watts or less do not require any of those measures. 

The factors that determine if 500 watts is enough include:

• The weight of the rider. Riders under 250 pounds will likely find 500 watts ample for most purposes. And there have been riders as heavy as 400 pounds who have found 500 watts to be enough when sticking to flat ground and small hills. But for bigger hills, heavier riders will likely find 750 watts a better choice.
• Your usual terrain. If you normally ride lots of steep hills, you may appreciate the power of a higher wattage motor. (But the less-is-more factor may also apply.)
• The weight of the bike, battery and motor combined. Bikes manufactured as ebikes are normally paired with a motor that can easily handle the weight of the bike plus the weight of the motor plus the weight of the battery plus the weight of an average size rider. But if you are converting a heavy conventional bike to an ebike, you may want the increased power from a 750-watt motor.
• The mileage range you want. Larger watt-hour batteries do provide more range, but a larger wattage motor draws down the battery more quickly and the weight of the larger battery increases the load on the bike, which also draws the battery output down. So in some cases where you want a large mileage range, you may be advised to use a smaller wattage motor — say 250 or 350 — to minimize weight and battery drawdown.
• How much assisting you want to do. Yes, with ebikes, we normally talk about how much the motor assists you as you pedal, but, if you wish, you can assist the bike by a) pedaling harder or b) shifting to a gear in which the current assist level of the bike is the most efficient.
• How fast you want to go. If you want to cruise near the 28 mph allowable speed in Class 3, a 750-watt motor will usually allow you to get there — assuming flat terrain and a rider of average size — whereas a 500-watt motor may top out at 25-mph.

To put all this in perspective, I’ll mention my two ebikes. The first one was a conversion. I installed a Bafang 750-watt mid-drive motor on my Trek 520 touring bike. I have since learned that Bafang makes smaller motors as well, but the U.S. seller of Bafang doesn’t offer them, and some friends who had already done conversions were using the 750-watt motor, so I went with it.

The bike gave me plenty of power, but the controller that came with the conversion kit had 9 assist levels, and while riding, I found that I rarely used any assist level higher than 5, and I more often rode in level 4, even while climbing steep hills. So, in effect, I was using only 40-50% percent of the available power. With the overall weight, I seldom pedaled that bike without at least minimal assist, but it still provided a range of about 65 miles on mixed terrain, though I could have increased that by buying a bigger battery.

I later purchased a Specialized Turbo Creo SL Comp E5 ebike, which in its bones is a lightweight high-end road bike. Its mid-drive motor puts out only 240 watts of power, and the bike has only three assist levels. I use all three, but the highest one only occasionally. Yet because of the bike’s minimal weight (even including the motor and battery) and the wide range of gearing, I often pedal some sections of my rides without employing the assist at all. The bike provides a range of about 80 miles of mixed terrain. 

I weigh 180 pounds, and on the Trek, I didn’t hesitate to carry extra items and even had a two-prong kickstand. On the Creo, I carry only essential items and eschew doodads like kickstands, but with that bike, less is indeed more.

My brother, who weighs only slightly more than I do, rides a conversion ebike with a 750-watt motor. On a recent ride together over rolling terrain, with me on the Creo, I noticed that while climbing out of the dips, he did so more quickly than I could, but that as soon as we topped the hills, I had no problem catching up with him.

All of this, coupled with the fact that the ebike classifications limit motor speed to 20 mph (or 28 mph for class 3), suggests that for most people in most circumstances, 500 watts is enough for their ebikes.

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Stan Purdum has ridden several long-distance bike trips, including an across-America ride recounted in his book Roll Around Heaven All Day, and a trek on U.S. 62, from Niagara Falls, New York, to El Paso, Texas, the subject of his book Playing in Traffic. Stan, a freelance writer and editor, lives in Ohio. See more at www.StanPurdum.com.