Watt vs kilowatt.hour

  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. sophiecentaur

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    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. russ_watters

    Staff: Mentor

    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. 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. sophiecentaur

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    Re: watt vs kilowat.hour

    Yep. Two issues here. 1. Believe the Physics. 2. Believe the arithmetic.
     
  7. russ_watters

    Staff: Mentor

    3. Believe the [mathematical] definition of the words.
     
  8. 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. sophiecentaur

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    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. 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. sophiecentaur

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    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. K^2

    K^2 2,470
    Science Advisor

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

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    Well, that's what he calls himself. It can't be 'Minister of Power'; the PM wouldn't like that.
     
  16. sophiecentaur

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    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. 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. sophiecentaur

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    If that's beer then we can have a party.
     
  19. 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. sophiecentaur

    sophiecentaur 13,914
    Science Advisor
    Gold Member

    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. 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
     
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