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About Electric Bike Motor Power Rating

Electric Bike Motor Power

The intent of this post was to share the maths and remove some of the mystery around the Power rating of the electric bikes you’d be looking at (like ours here) but I found a guy who did a better job than I would have.

Here is the link to his post.

But before you bounce away from us I’ll summarise and extend a little for you under this link (which opens in a new window whereas the picture will leave this page..)
http://www.ebikeschool.com/myth-ebike-wattage/.

The Myth of Ebike Wattage

Power is a measurement of the ability to do work in Physics and is a Joule per Second. Yay.

In Electric bikes the definition is based on electricity and is 1 Watt = 1 Volt x 1 Amp. A Volt is the amount of push in a circuit and an Amp measures the number of electrons travelling per unit time. More yay.

The thing to know is that the number given to an electric motor is a bit like the number given to Audio Speaker ratings. There is the amount a motor or speaker can handle momentarily and there is the amount it can handle continuously.

In Speakers there is the PMPO rating (Peak Momentary Power Out) and it is different to the RMS (Root Mean Square) in that the PMPO is the amount of current the speaker can handle for just a tiny time. Like a drum beat. RMS power on the other hand is the amount the speaker can handle continuously. So the RMS number may be significantly lower than the PMPO number yet represent a more powerful speaker.

The same is true for electric bikes. The power at any point in time is determined by the amount of current the controller of the bike is feeding to the motor multiplied by the voltage of the motor.

This means the power use of the motor on the way up a hill will be much higher than when you are cruising along.

So the 250 Watt motor rating could mean the most it can handle, or it could mean the amount it can handle all day and this varies on the manufacturer’s decision. In truth as mentioned in the article above, the actual amount the motor can handle is well above the amount written on the motor in general.

Power vs Battery Life

So now that you know that power is Voltage times Current, and given that a battery is basically a way to store a whole heap of electrons that are negatively charged that want to get around a circuit to the positive side of your battery, then perhaps you can understand that the power your bike is generating comes from it using up electrons and that once they run out the bike stops.

This is why batteries are measured in AmpHours. Amp is a measurement of electrons rushing past per second. Hour is obviously a measurement of time so a 12 AmpHour battery is a number that tells you that the battery can deliver 12 Amps of current for 1 hr, or 1 Amp of current for 12 Hours, or 2 for 6 etcetera. It is a measurement of the battery’s capacity.

A larger power motor on your bike that you run at maximum power will tend to use up your battery the quickest.

But just like a big motor in a car, if you drive the car gently you can save fuel, ride your bike with the boost not set through the roof will tend to give you maximum range on your bike.

Motor Location

The other thing about motor power is that where the motor is situated on the bike impacts how much gets to the road.

A motor in the rear hub of the bike will put a lot more of its power to the road than a bike with the motor mounted in the crank at the middle of the bike.

Conclusion

The motor power you read should be taken as an indication of what the bike can do, not an absolute measurement. Don’t worry about it too much when choosing your bike. You are better off going for a ride and deciding that way.

In general a bike with a higher voltage battery (48V compared to say 36V) will tend to have more actual real world grunt.

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