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  1. #1
    Junior Member
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    Jan 2012
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    Differences in motor watts?


    I built a 36v20ah 800watt bike. Runs great. Faster than I like to ride downhill, yet good speed going uphill. I'd like to build another bike but better if possible. I don't really understand the difference mixtures of motor watts to battery volts.

    Can someone explain the basic difference in bicycle motor watts? Say for example, I have a 36V lifepo battery and used it on a 500watt motor and then a 800watt motor. What will be the difference in performances? Faster speeds one way or another. Better hill climbing?

    Reason I ask, is that i see very little difference in prices from one motor to the next. Oh... maybe 30 or 50 dollars difference. So if a 250w kit is $200, 500w kit is $230 and a 800w is $250, etc... then why not buy the bigger motor?

    Further confusing the matter, I see kits with mixed battery volts vs motor watts. I see 250 watts with 48volt battery/controller, then I see 1000watt motor for 48volt battery/controllers (and many combinations in between).

    Lastly, I read on an e-bike retailer's page that "ah" translate to "mph", i.e. 10ah=10mph, 20ah=20mph, etc... is this roughly correct or not? Seems my 36v20ah bike goes roughly 20 mph (+or-) which supports the retailer's assertion.


  2. #2
    Senior Member
    Join Date
    Aug 2011
    Western Canada
    My Bikes
    E+ kit, Tidal Force, BIONX
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    I will try,
    Power is measured in Watts=voltage times current, so from basic math if you increase voltage you use less current for the same power, right?
    Power is a rate at which electrical energy is converted into mechanical energy inside your motor.
    Power is a consumption of el. energy, the more rated power of the motor the more el. energy is going to be consumed, so 800W motor is going to consume twice the energy than 400 motor.
    Performance will improve on 800W motor versus 400W one but energy from battery will be depleted twice that fast. So yes faster spped but for shorther time.
    Amphour /Ah/ has nothing to do with speed, it is capacity of battery= how long before battery is deplated.
    From 20Ah battery you can draw in theory 20Amps for 1 hour or 1Amp for 20 hours or 5Amps for 4 hours, etc.=battery capacity.

    Last edited by powell; 02-01-12 at 09:33 PM.

  3. #3
    Senior Member
    Join Date
    Oct 2010
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    Micro is basically correct...

    However, the motor rating in watts is a misnomer...

    Most of the time, the eBay auctions you see are done by sales people and not the technicians..

    Motors SHOULD be rated in the amount of watts continuous that it can handle..

    Below is a copy and paste of another post of now to help clarify..

    Hub motors are broken up into 2 types...

    1.) Small Geared
    2.) Larger Direct Drive

    Small geared are usually in the 250-350w range.. They are lighter, more torquey, but have less top end.
    Direct drive are usually in the 500-1000w range.. They are heavier (not by much) and are capable of much faster speeds...

    To speak to the "wattage" of a motor is a misnomer...
    The wattage is USALLY set by the controller that runs the motor..
    (i.e. This is no difference between a 500w and a 1000w "motor", it's the controller that provides more power (amps) and is the controlling factor..)

    Most geared motors will handle up to 500w, after that, you really need a direct drive..
    DD motors can usually handle 1200w continuous, without issue..
    The more watts you start pushing into the motors, the more that the energy put in is transfered to heat.

    As a general rule...
    24v will get you 15mph top speed (usually 250-350w)
    36v will get you 20mph (350-750w)
    48v will get you 25mph (750w and up)

    The equation is V olts times A mps = W atts or VxA=W

    As you will notice, all of those figures equate to 10-15 amps on average...
    (24v X 10a = 240w, 24v X 15a = 360w)
    (36x X 10a = 360w, 36v X 15a = 540w)
    Etc, etc...

    So a 36v 15a controller will provide up to 540w (36v X 15a)
    a.k.a. A 36v 500w motor
    If the controller was a 30a one, then I would be (36v X 30a) = 960w
    Or a 36v 1000w motor.

    Hope that all makes sense.
    Last edited by Sangesf; 02-02-12 at 12:25 PM.

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