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Power calculations aren't this easy are they?

Discussion in 'Power Electronics' started by Braeden Hamson, Aug 7, 2019.

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  1. Braeden Hamson

    Braeden Hamson

    Feb 18, 2016
    So uhh, I realized today that some of my basics are shaky. Lets say you have a device that consumes 10W of power, and it runs for 5 hours. Did it consume 50 watts? What if it ran for 5 seconds? Now lets say you have a resistor that is 1000 Ω, running at 10 V. P = V^2/R so 10V^2/1000Ω = 1 W. If this resistor runs for 5 hours, is that 5 watts consumed?

    I feel like I'm over thinking this.
  2. Bluejets


    Oct 5, 2014
    On DC......
    10W for 5 hours is 50Wh

    10 x 10 /1000 = 0.1W

    0.1W x 5 = 0.5Wh
    Braeden Hamson and davenn like this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Power is given in Watts, Energy is given in Wattseconds or multiples like Watthours.
    Regardless of how long the device is operated under the conditions you stated, the instantaneous power is 10 W. The energy dissipated depends on how long you run the device, see @Bluejet 's calculation.

    Simply look at the electricity meter in your home: as long as you run an appliance the meter counts up. It meters the energy, not the power. How fast it counts depends on the power. Compare a small light bulb and a tumbler...
    Braeden Hamson likes this.
  4. Nanren888


    Nov 8, 2015
    So to recap.
    Watts measures a rate of flow of energy. A Watt is a Joule per second.
    In many electrical things watt-hour is a more convenient measure, as above, as a Joule is not a lot of energy in the power world.. A Watthour is a Joule per second for an hour, or 3600 Joules.
    Something heating at 10 watts means it absorbs 10 Joules per second.
    So for the example of an electric jug heating water, the power (watts) tells you, for any particular fixed amount of water, how quickly the water will heat.
    For a partcular amount of water, with all the energy going into heating the water, the total energy absorbed (watts times time) will tell you how hot that amount of water gets in the time you heated it.
    So, more Watts, means doing things faster.
    Generally, of course, you pay someone for the total energy used, Watthours, not Watts.As suggested by what Harald Kapp said, the higher the power, the faster it counts Watthours, the faster the bill increases.
    Braeden Hamson likes this.
  5. Braeden Hamson

    Braeden Hamson

    Feb 18, 2016
    I knew I was overthinking it. The key point you guys helped me realize was I can't say it used 50 watts. I have to say it used 59 Wh. But I can say it's using 10W. Thank you all
  6. Kabelsalat


    Jul 5, 2011
    No you didn't overthinking at all. Let me help you:
    Say you have a house - the base voltage is 230V (or 110 if in US). In the house you have say 15 electric heaters, each have a thermometer set at a certain temperature.
    Then the voltage decrease to say 190V (91V in US). What will happens to the overall wattage consumption of that house when the voltage drops and staying low ?

    This isn't very helpful, really
  7. davenn

    davenn Moderator

    Sep 5, 2009

    this isn't correct … there is no voltage drop like that
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