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Thread: Understanding the kilowatt hour

  1. #1 Understanding the kilowatt hour 
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    1 gallon of gasoline generates 115k BTUs
    To generate 115k BTUs electrically, 33.7 kWh are needed


    This is the equation from which the EPA determines MPGe (miles per gallon equivalent) for electric cars. I know the kilowatt hour (kWh) is the standard unit of billing in American households but from a more logical standpoint why are we so stuck to the kilowatt hour and not just the kilowatt?

    If 33.7 kWh generates 115k BTUs, does that mean it could generate that heat in 1 hour using 33.7 kW or 16.85 kW in 2 hours?

    And to use an example from Wikipedia, if a person having a mass of 100 kilograms climbs a 3-meter-high ladder in 5 seconds they are doing work at a rate of about 600 watts. Can I multiply that 600 watts by 12 to say 7,200 watts consumed in one hour or simply 7.2 kWh?


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    Quote Originally Posted by smokemonster View Post
    1 gallon of gasoline generates 115k BTUs
    To generate 115k BTUs electrically, 33.7 kWh are needed


    This is the equation from which the EPA determines MPGe (miles per gallon equivalent) for electric cars. I know the kilowatt hour (kWh) is the standard unit of billing in American households but from a more logical standpoint why are we so stuck to the kilowatt hour and not just the kilowatt?
    Kilowatts cannot be compared to BTUs in a gallon of gasoline. One is the rate of producing energy, the other is a quantity of energy.
    If 33.7 kWh generates 115k BTUs, does that mean it could generate that heat in 1 hour using 33.7 kW or 16.85 kW in 2 hours?
    Yes
    And to use an example from Wikipedia, if a person having a mass of 100 kilograms climbs a 3-meter-high ladder in 5 seconds they are doing work at a rate of about 600 watts. Can I multiply that 600 watts by 12 to say 7,200 watts consumed in one hour or simply 7.2 kWh?
    Yes, but I think you will find that a person is not able to do work at the rate of 600 watts for 12 hours straight.


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