How Big of a Motor Do I Need for Electric Bike?

There’s no single right answer to the question, “how big of a motor do I need for electric bike use?” It depends on the weight of your bike, the terrain where you ride, and how much assistance you want. Choosing the best motor rating for your e-bike carries some profound benefits, including increased speed. You’ll also be able to ascend steep hills more easily. However, most KW-sized electric motors are relatively heavy and require higher voltage systems with a greater battery capacity. In many places, eBikes with more than 1 KW may be legally classified as motor vehicles. Depending on the laws in your area, this means a motorcycle license and helmet may be required. Have a look at our buying guide on the best electric bikes to learn more.

How E-Bike Motors Are Rated

Electric Motors can be rated in different ways, making it sometimes confusing to compare them.

Input vs. Output

You might see electric motors listed by volts, Amps, Watts, or even Horsepower or lb-ft. Many brushless DC motors for electric bikes are rated in terms of input voltage.

Volts and amperage describe the input–the electric current that goes into the motor. More voltage means a higher frequency current and the motor spinning faster, while more amps mean more current, and the motor spinning “harder.” Put the two measurements together, and you get Watts, a measure of power. Most off-the-shelf electric motors list their input, Watts. Output depends, in turn, on input plus motor efficiency.

For example, a motor with a rating of 1000 Watts continuous power, given a 90% efficiency, will have an output of 900 Watts. This is equivalent to the amount of horsepower it can send to the wheel of your bike.

Power and Torque Ratings

Just like internal combustion engines, electric motors can have equivalent power ratings but very different sizes and uses. A 750-watt motor can be small and spin very fast, as RC model airplane motors do, or it can be much larger and spin more slowly, as winch motors do.

Picking the right motor for your eBike depends on knowing how much higher-speed power and how much lower-speed torque your electric bicycle will need.

The necessary power depends mostly on the speed, drive system type, and gearing of your electric bicycle. Torque needs depend on weight, terrain, and whether you’re using throttle-assist or pedal-assist only.

Top speed is related to power level. As a general guide, a 15 to 18-mph eBike needs about 250W, while a 20-mph eBike needs around 500 to 750W.

Types of Motor and Drive Systems

Electric bicycle motors may also be classed according to how the drive system is set up on the bike: motors can be mounted in the hub of the front or rear wheel or near the crank and can be direct-drive or geared.

Direct-Drive Hub Motors

Placing the motor around the hub of the wheel and requiring no gearing, these are a popular choice for DIY electric bicycle kits. However, the motors need to be relatively large to generate enough torque at a very low RPM to work without reduction gearing. The additional weight on one wheel may also make the bike feel less balanced than a standard non-electric bicycle. But no matter the type of motor or its placement, always make sure it is covered in water-resistant material. A water-resistant motor casing is necessary in order to safeguard your e-bike from riding in the rain.

Geared Hub Motors

Geared hub motors can be more energy-efficient than direct-drive motors. Unlike direct-drive, these allow for a clutch that lets the bike freewheel when decelerating or when assistance is not needed. The gear systems can add complexity and cost, however.

Mid Drive Motors

Mid-Drive systems mount the motor near the crank and use it to drive the chain. This set-up provides advantages in terms of weight distribution and the ability to change gears to make the most of the motor power available. Mountain bikes with electric assist are usually mid-drive designs. Mid-drive motors can be just about any power level, although with high torque motors you’ll want to use a controller with torque sensors to help prevent damaging the chain and sprockets. Mid-drive electric bikes can also provide regenerative braking, which is applied on electric bikes that will charge when you pedal.

Laws about Electric Bike Motor Sizes

Local regulations may classify eBikes differently according to motor power or size. For example, many European jurisdictions only allow eBikes with 250W or less to use a bike lane. US federal law defines an eBike as having a motor with 750 Watts of power or less (1 horsepower) and a 20 mph top speed when on throttle assist alone. In general, electric bikes with more powerful motors may be subject to the laws applicable to mopeds, dirt bikes, and other small motorized vehicles.

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