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Watt / Watt-Hour

  1. Jun 25, 2008 #1
    A light bulb which is 20Watt, is run for 2 hours
    equals to 40Wh, which mean 40Watt used in an hour

    is the concept correct?

    2.Assume above was correct.
    The 20Watt is powered by a battery which is 200Watt.
    This battery can last for 5 hours.

    IS the concept above correct?

    A wind turbine with 8Watt is connected to the battery(200Watt), to charge the battery before the it connected to the light bulb.
    The wind turbine is run for 10 hours, what is the watt-hr of the turbine, and the total wattage in the battery after 10hrs.

    Last edited: Jun 25, 2008
  2. jcsd
  3. Jun 25, 2008 #2


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    No, 40 Wh does not mean: 40 Watt used in an hour, that would be 40 W/h. Instead, 40 Wh means: enough energy to give 40 Watt constantly during one hour. Or, 20 W for two hours. Or 1 W for 40 hours. So if you would have a battery of 200 Wh, you could connect a 200 W machine and it would take exactly one hour for the battery to deplete. If you would connect a 50 W device instead, the battery would last for 4 hours.

    Remember, that Watt is actually Joules / second, so energy per time. So if e.g. a light bulb is 20 W, that means that it uses 20 J every second. If you leave it on for an hour, it will use (20 J/s) * (3600 s) = 72 kJ.
    When using Wh, you are actually multiplying this energy per time by a time again, so you will get an energy. You can even calculate how much a Wh is:
    1 Watt * hour = 1 Joule / second * 1 hour = 1 Joule / second * 3600 second = 3600 Joule
    So if you get confused, you can just convert from Wh to J, of which you (hopefully) will always know what kind of quantity it is.
  4. Jun 25, 2008 #3


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    A Watt-hour is a unit of energy.
    A Watt is a unit of power (rate of energy consumption)


    1. Not quite. A 20W bulb run for 2 hours will use 40Wh of energy. It's meaningless to say "40W used in an hour", because you're not using up Watts, they're just happening constantly. A vague analogy would be if you're travelling at 20mph for 2 hours, you'd cover 40 miles. But you wouldn't use 40mph in an hour.

    2. No, this is not correct. Your battery wouldn't hold 200W, since this is power; not energy. If your battery contained 200Wh of energy, then your 20W bulb would light for 10 hours. (In reality it wouldn't, because your battery would go flat, but you get the idea). An analogy to your premise would be "the car travelling at 20mph has 200mph of fuel in the tank, how long can it drive for?" which is clearly senseless.

    3. Since your battery can't be described as 200W, and that batteries don't store power (they store energy), this question is meaningless. A turbine doesn't have a "watt-hour"; a turbine has a power rating (eg 8W). The watt-hour bit depends entirely on how long it's run for, at what power. So, a correct statement would be:

    "A wind turbine producing 8W, when run for 10 hours, will produce 80Wh of energy".

  5. Jun 25, 2008 #4


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    I wouldn't normally pick this up, but it's wrong, and it's relevant, so:

    No. 40 Watts used over a period of 1 hour would be 40 Watt-hours, not 40 Watts per hour.
  6. Jun 25, 2008 #5
    thanks for your quick and useful reply.

    But for this wind turbine, http://www.reuk.co.uk/Envirotek-V20-VAWT-Generator.htm
    It produces 12V, and 3Ah/day under 10mph
    It equals to 36Watt-Hour /day under 10mph
    So it requires 1.389 Days to fully charge the 50Watt-Hour battery.(Ideally)

    Am I correct?
  7. Jun 25, 2008 #6


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    All that is correct except (perhaps) the last line. Batteries typically aren't rated in "watt-hours", they are rated in amp-hours. 50 watt-hours for a 12 V battery would only be a little over 4 Ah, which is pretty small for a 12V battery. Typically, people use power tanks for this kind of thing and they run 7-17 Ah: http://www.sears.com/shc/s/p_10153_12605_02871486000P?vName=Automotive
    Last edited by a moderator: Apr 23, 2017
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