E-Bike 101: Understanding Power

Electric motors are complicated! If all the wattages, amp hours, and volts is not something you understand, our simple guide will help you understand how an electric motor works so you can compare different models and brands accurately. First let's start off by clearing up the confusion caused by others.

 

 

Myth #1 - Power is determined by the motor.

Many of our competitors advertise the wattage of their motors, this would be similar to advertising the maximum speed of the tires on your car.  Just because you put a 200mph tire on a geo metro doesn't mean you now have a corvette!  

Myth #2 - Power is determined by the battery.

A larger battery will not increase the power output by the motor, but only give you more range.  You may be wondering why bigger motors require bigger batteries, a battery must be able to output enough current to meet the demands of the motor so a bigger motor may require a larger battery, but further increases in capacity will not increase the motor power.

Fact: Power is determined by the controller!

The simple equation to determine the approximate power output of an electric bicycle is to multiple the voltage of the system (V) times the continuous current rating of the controller (I) times the overall efficiency of the system (around 80%).  There are expensive mechanical testing apparatuses that can calculate the exact power output, but it is much simpler to estimate the output with the motor efficiency.
IE: 48V System, 15Amp continuous controller = 48 * 15 * .8 = ~576 Watts output.

What about peak watts?

Peak watts is an incorrect term that doesn't apply to electric bicycle controllers. Older brushless motors where peak watts was relevant used a different type of controller, however, they were not designed for use with lithium ion batteries that have current limits. Modern electric bicycles use a different type of controller that allow you to maximize battery output. 
For companies that list peak watts, they typically multiply the controller current by the voltage, #1 in the picture above. For the same example as above you would have
48V * 15 Amp = 720 Amps

Which power are people advertising?

Historically manufacturers advertised the output of the motor (#2 in picture above) as this is what laws regulate. Recently some brands started advertising the power that comes out of the battery (#1 in the picture above) because this gives you a much larger number as it does not include the efficiency of the controller and motor.  It is also easily calculated as you can just multiple the voltage by the current as shown above. It is NOT, however, the power output of the motor.

Which do you advertise?

We advertise power output, not the input.  Our 500W output motors have the same power as many of our competitors 750W systems as they are advertising #1 in the diagram not #2.  This may not always be the case as people can change their advertising and there is no standard that manufactures must adhere to like the automotive industry.

The Proof Is In The Pudding

Here we have our VeeGo Fat Tire with 500 Watts of Output against our competitors 'Powerful 750 Watt' Bicycle.  Not only is the VeeGo at an alleged 250 watt disadvantage, the rider also weighs 30 pounds more! If you went strictly by the advertised wattage the competitors bike should be 70% faster!

Music by BenSound.com
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2 comments

  • Hi Tom! From my perspective the issue is people assume this ‘Peak Watt’ is some short term boost, for example if I hit the throttle I will get 720Watts of power for a few seconds as some sort of jolt regardless of the motor RPM. Unfortunately this is not the case, and doesn’t take efficiency into account. If you are going very slow efficiency may only be 40% so the actual output would be 288 Watts. If you are at the upper end of efficiency say 82% then the output would be 590 Watts, but there is no short term 720 watt output. Also the power draw from the battery would be consistent, but the output changes as you move to a more efficient motor speed.

    Jason
  • I could be wrong, but I believe in “What about peak watts?” above; the sample calculation “48V * 15 Amp = 720 Amps”, resultant should be 720 Watts if employing Ohm’s Law.

    Thomas Stanziale

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