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Watt vs kilowatt.hour

  1. Nov 27, 2012 #1
    Hello,

    getting confused about watt vs watt.hour

    in the Watt's wiki page, an example is given on how to calculate wattage (power):

    So, (100kg x (9.81m/s2) x 3m) / 5s = 588 kg.m2/s3

    this system needs aroud 600 watts of power to accomplish its task. ok.

    Then, lower in the page, an example is given on how to calculate watt.hour(energy):


    Now, if I go back to my first example, it's being done in 5 seconds for 588,6 watts

    if it was done in 1 second, it would be 2943 watts

    if it was done in 3600 seconds, it would be 0,8175 watts

    So, if I do the work on a 1 hour (3600 sec) time span, can say I use 0,8175 watt.hour?

    And if I lift the thing in 1 second, but constantly, for 1 hour, do I use 2943 watt.hour?

    This is confusing me a little, so I guess my question also is a little confused. I looked many places where it was promised that this confusion would evaporate after reading their explanations. Now I'm here because I'm still a little lost.

    thanks
     
  2. jcsd
  3. Nov 27, 2012 #2

    sophiecentaur

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    Re: watt vs kilowat.hour

    It may help to ignore the actual figures for a start. The Watt is a unit of Power - the rate of doing work or using energy. The kW hour is a unit of Energy - just like the more basic Joule. If you read each problem / example carefully you will always see whether Power or Energy is what is needed.
    Power is Energy / Time
    Energy is Power X Time
    Hold that in your head and the numbers will look after themselves.
     
  4. Nov 27, 2012 #3

    russ_watters

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    Re: watt vs kilowat.hour

    Welcome to PF!

    Your confusion isn't affecting your ability to do the math, so I'm not sure what there is we can do to help other than to tell you to trust your understanding, because you understand correctly.

    ...Or ask us another question...
     
  5. Nov 27, 2012 #4
    Re: watt vs kilowat.hour

    NO need to confuse over it. Just Take it easy. I try to make it clear.
    Actually the rate of work done is called power. I mean for calculating the power you must know the work done. ... Work done per sec is called power and measured in Watt. Work done per minute is also power. Work done per hour is also power but measured in Watt.hour.
    Actually Watthour And killowatthour is represent the power produce by a machine in 1 hour.

    Suppose a machine produce 1500watt power in one sec. then it will produce 1500X60 watt in a minute and 1500x60x60 in a hour or i can say 1500x60x60/100 Kwatthour (KWH)
    hope you understand
    thanks
     
  6. Nov 27, 2012 #5

    sophiecentaur

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    Re: watt vs kilowat.hour

    Yep. Two issues here. 1. Believe the Physics. 2. Believe the arithmetic.
     
  7. Nov 27, 2012 #6

    russ_watters

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    3. Believe the [mathematical] definition of the words.
     
  8. Nov 27, 2012 #7
    Re: watt vs kilowat.hour

    No, they represent energy and not power.
    A watt-hour represents a specific amount of energy (3600 J) no matter in what time is this energy used or transformed.
    Even saying "power produced in 1 hour" is meaningless.
    The machine "produces" energy. Power shows you how fast is done, not necessarily how much of it.
    1 watt -hour of energy may be "produced" in 1 second or 1 billion years. It's still 1watt-hour.
     
  9. Nov 27, 2012 #8

    sophiecentaur

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    Re: watt vs kilowat.hour

    I agree.
    I find people's determination to use the wrong terms, in this way, very annoying. It's as if they really don't get it. People who get aerated about the 'offside rule', as if it's actually important. But they will use scientific terms interchangeably like some poet who can't bring himself to use the same word twice in a sentence and scrabbles around to find a shoddy equivalent. The fact is that there is, with very few exceptions, only one word for each scientific concept. It just does not help anyone to have the alternative (wrong) terms used instead. It doesn't make the subject any more approachable - just the reverse.

    I heard some clown of an energy minister on BBC Radio, recently, talking about "storing Power"!!! What hope do we have?
     
  10. Nov 27, 2012 #9
    Re: watt vs kilowat.hour

    Well, for a minister I think it's OK. He's not trying to learn or teach (thanks God) physics.
    In common language, power is used so often for "energy" that maybe you should close you eyes. It's a "power station" or "power plant" after all. At least in North America.
    A car factory produces cars which are stored in parking lots. So what does a power plant produce (and store)?:confused:
     
  11. Nov 27, 2012 #10

    sophiecentaur

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    I clearly made the mistake of hoping that he should know what he's talking about.
    If he's as ignorant as that, how can he distinguish between sense and nonsense when he's being sold some idea by a flash salesman? The distinction could be highly relevant to making a good decision.
    For instance, what use is a 5MW wind turbine if it only operates for 30 days per year?
    (You wouldn't be confused, by common language, yourself would you??? :devil:)
     
  12. Nov 27, 2012 #11

    K^2

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    Sell excess back to electric company in these 30 days, use power from the grid for the remaining 335 days. At 5MW, you are still going to be making a net profit.

    :p
     
  13. Nov 27, 2012 #12
    Oh, I did not pay attention that you said energy minister.
    This makes some difference.
     
  14. Nov 27, 2012 #13
    And how is the electric company going to store all the power it buys from people?:smile:
     
  15. Nov 27, 2012 #14

    sophiecentaur

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    Well, that's what he calls himself. It can't be 'Minister of Power'; the PM wouldn't like that.
     
  16. Nov 27, 2012 #15

    sophiecentaur

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    Hmmm
    Not sure you could actually fit a 5MW turbine on your roof. I was thinking more of those big devils out at sea. They are owned and run by the Elec company.

    Did you consider the capital cost of one that big?
     
  17. Nov 27, 2012 #16
    Kuin,

    Here is an analogy for you.

    The bottled water factory produces 8,000 gallons per day.

    I can store 10 gallons in my pantry.

    See the difference between "gallons per day" and "gallons"?
     
    Last edited: Nov 27, 2012
  18. Nov 27, 2012 #17

    sophiecentaur

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    If that's beer then we can have a party.
     
  19. Nov 27, 2012 #18

    CWatters

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    A lot of the confusion comes because the energy companies use kWh as a measure of energy which it is (because its power * time) but it's not a basic unit like the joule. To make matters worse you sometimes find people talking about "kWh per hour" which is the same as saying kW.
     
  20. Nov 27, 2012 #19

    sophiecentaur

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    Same quantity but one is time averaged (e.g. including a thermostat operating) and the other is instantaneous. That first one may look hideous but it could have relevance somewhere.
     
  21. Nov 27, 2012 #20
    ok, thanks for all the replies, I think I got a glimpse at something, the fog seems to be thinning out.

    So, is my understandng right if say :

    I attach a mass of 102 grams (0,102 kg) to a 1 meter long string that is wound around a shaft that turns a small electric generator. (assume a perfect no loss system)

    When I let the weight go down, it would ''produce'' a power of 1 Watt, or 1 Joule per second.

    because [itex]0,102kg\times[/itex][itex]\frac{9,81m}{1s^{2}}[/itex][itex]\times1m[/itex] / 1s = 1 Watt.

    BUT, this works only if the duration is exactly 1 second. How do I make this happen? If I get around the problem and say I lift the thing in 1 second, now it's easy, I control that, but if I have to let gravity do the work, how can you control the duration?

    Then for watt-hour:

    Say I have a very very long string and the generator is up the highest cliff. I let the 102g mass go down for 1 hour. Will it produce 1 watt-hour of energy?

    thanks
     
  22. Nov 27, 2012 #21
    nasu,
    Sorry, I mis-directed my last post at you. Sorry, I have updated it.

    Kuin,
    I think you have it, your calculations look correct.

    You ask how would you control your gravity-weight-generator mechanism in order to have it produce at a steady rate of 1Watt? This would require some kind of speed regulator, the generator would have to be spinning at a constant rate.

    The real question is, why would you want/need to do this? In reality, the weight would eventually reach a terminal velocity where the reaction force from the spinning generator was equal to mg, but prior to this, the output rate in watts would not be constant.

    In other words, power is, or can be, a function of time in which case finding total energy would require integrating over time vs. multiplying by time.
     
  23. Nov 27, 2012 #22
    You are still going around in circles.
    1 watt-hour is 3600 J.
    The weight of your mass is about 1 N. It has to go down 3600 m for the work of gravity to be 3600J or 1W-hour. No matter how long it takes. Work= F*d See? no time in here!
    How fast it goes down determines the power (in watt) and not the energy in Watt-hour.
     
  24. Nov 27, 2012 #23
    1 joule is equal to 1 watt second. (Ws)

    Does that help? I didn't bother to read all responses yet.
     
  25. Nov 28, 2012 #24
    So,

    in the end, what I'm trying to do is to find a way to get a clear image of the energy used by a typical household over the course of 1 year. A north american typical household of 4 people uses around 10 000kwh every year.

    but it's hard to figure out what that means so I'm trying to convert that to something we can understand instinctively. I think weights moving up or down give the clearest 'picture' for that purpose. So I'm trying to figure out what 'size' of a weight would have to travel how high to 'work' 10 000kwh.

    As for the weight itself, water is a good pick, people can easily imagine what a liter of water 'feels' like, and it is also handy since 1 liter of water has a weight of 1kg.

    so, is this right:

    10 000kwh = 36 000 000 000 N-m = 3 670 978 356 kg-m

    so, 3 670 978 356 kg going down 1 meter would produce 10 000 kwh

    or 36 709 783 kg going down 100 meters

    so we need to picture a cube of water containing 36 709 783 liters of water going down 100 meter.

    cubic root of 36 709 783 is 332 liters(10cmx10cm,x10cm) of edge for the cube, that is

    332 x 10cm = 3323cm = 33meters

    So a cube 33 meters of side, filled with water, going down 100 meters represents the energy usage of 1 north american household... 33 meters is about the height of a 10 story building. 100 meters would be a 30 story building.

    I must be wrong, this would be crazy! this seems a lot.

    is this right?
     
  26. Nov 28, 2012 #25
    Yes, I think you're right :smile: -- I get the same answer. It isn't crazy, it's just that gravity isn't very strong. Big hydroelectric dams need rivers with alot of flow and alot of head.
     
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