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Thread: kW*h and dollars per year

  1. #1 kW*h and dollars per year 
    Forum Bachelors Degree Demen Tolden's Avatar
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    I seem to be having a real difficult time doing a conversion that I confess I do not understand very well. What I am trying to do is convert x watts or x joules into dollars per year using a conversion factor of 3.9 cents per kW*h.

    I know I am surely wrong, but I feel like I am trying to convert two dimensions into three so I kind of feel a little frustrated. I'll start from the beginning of my thinking process.

    Lets start with Force. Force is mass multiplied by acceleration, or:

    The units for this are:

    or 1 newton, N

    Next, lets measure force over a distance of a meter and call this Work. The units for work are:

    or 1 joule, J

    Next lets measure the amount of work we can do within the time interval of a second and call this Power. The units for power are:

    or 1 watt, W

    I feel I completely understand this so far, but after this I get lost. Aparently the next unit, the one that shows up on the utility bill is kW*h. What is this? This doesn't make sense to me. I could understand kJ*h as in an alternative for W, but not kW*h.


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  3. #2  
    Forum Radioactive Isotope MagiMaster's Avatar
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    If I understand right, watts is an instantaneous measure of work. You have to multiply that by how long you work for to get the amount of actual work done. (Note, I may not be using work in the proper, scientific sense.) So working at a rate of one kilowatt for one hour converts one kilowatt-hour of energy. Wikipedia mentions that kilowatt-hours is comparable to joules. I think 1 joule is equal to 1 watt-second.

    Edit: Oh, and I think power is the rate of change of work, not the work done over a second. At least, again, according to Wikipedia. Admittedly, I'm not that great at physics.


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  4. #3  
    Forum Bachelors Degree Demen Tolden's Avatar
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    Quote Originally Posted by magimaster
    If I understand right, watts is an instantaneous measure of work
    Quote Originally Posted by wiki
    The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second. It measures a rate of energy conversion.
    Quote Originally Posted by magimaster
    I think power is the rate of change of work, not the work done over a second.
    Wouldn't any measured quantity over its measured period of time be a rate of change?
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    What you are paying for on your electric bill is energy (work). A watt is a unit of power (rate of energy usage), so your bill is based on the power multiplied by the time. A watt-second is a joule. There are 3600 watt-seconds in a watt-hour. There are 3,600,000 watt-seconds (i.e., joules) in a kilowatt-hour.

    Suppose you have a hair dryer which consumes 1000 watts. That's a kilowatt. If you run it for an hour you have used 1 kilowatt-hour of electricity.
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    Quote Originally Posted by MagiMaster
    If I understand right, watts is an instantaneous measure of work. You have to multiply that by how long you work for to get the amount of actual work done. (Note, I may not be using work in the proper, scientific sense.) So working at a rate of one kilowatt for one hour converts one kilowatt-hour of energy. Wikipedia mentions that kilowatt-hours is comparable to joules. I think 1 joule is equal to 1 watt-second.

    Edit: Oh, and I think power is the rate of change of work, not the work done over a second. At least, again, according to Wikipedia. Admittedly, I'm not that great at physics.

    They often put it, one watt is equal to one Joule second. And then they put it One Joule is equal to one watt second, on the next page.

    Any measure of power or energy needs a time period listed with it, if applicable, or you risk confusion. BTU's are often listed with a time value. If you just want to know the volts or amps flowing through a circuit then you do not need a time value.

    But if you are measuring a total of the power or energy put through a circuit over a given time. Then you should include a time value. For scientific purposes, to avoid confusion.



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    Here are some actual conversions and definitions.


    Joule is a unit of work = 10^7 ergs.

    Joules= (volts^2 X seconds)/ohms = watts X seconds = amperes^2 X ohms X seconds.

    Watts are a unit of electrical power. The unit of electric power = 10^7 units of power in the C.G.S. System. It is represented sufficiently well for practical purposes by One Joule per second.

    Watts = Volts X amperes = amperes^2 X ohms =volts^2 / ohms.

    The way they write them is a bit confusing. However they are equal units with a similar time value for both.

    I had mentioned on the Internet years ago that the computer surge suppressors rated in Joules may not give you much protection. But many have a misconception of the Joule and Watt. And they denied that.

    There were some surge suppressors rated at 500 Joules. It is amazing what you can sell people that don't know.




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    William McCormick
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  8. #7  
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    Quote Originally Posted by Harold14370
    What you are paying for on your electric bill is energy (work).
    I dunno about work. But I do know that I'm paying money to some guys to ride around in yellow trucks.
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