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Converting wattage to KWH?

  1. Aug 20, 2008 #1
    I'm attempting to calculate the amount of power (in KWH) produced by a water turbine based on x litres of water falling x metres vertically through a tube of x cm diameter

    So far i've managed the following:
    H = height (distance fallen) (m)
    Time = Secs
    R = Radius of the pipe (m)

    Velocity (m/s) = (2 x H x G) ^ 0.5
    Volume / Sec = Velocity x Time x Pi x R^2
    mass (kg) / Sec = (Volume / sec) x 1000
    Energy = (Mass x Velocity^2)/2 = wattage generated

    My questions is.... how do i convert this wattage generated in KWH generated?

    Once I have this am i correct in thinking I can do:
    (KWH generated / 3600) * (time taken for x litres of water to fall through the pipe).
    To calculate the KWH generated by the x litres of water falling?

    Any help / guidance anyone could give me would be greatly appreciated. - As you can (maybe) tell i'm by no means a physics graduate but i do have a basic understanding.

    Any help anyone could give me would be greatly appreciated.

    Many Thanks

    P.s For the purposes of this i'm happy to work with a turbine having 100% efficiency.
  2. jcsd
  3. Aug 20, 2008 #2


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    Power isn't measured in kWh; that's a unit of energy.

    Energy (in Watt-hours) = power (in Watts) * time (in hours).

    So, if your turbine produces 100kW (power) constantly for 3 hours (time), you will have produced 300kWh of energy.

    The short way of finding out the amount of energy available in your example would just be to use:

    Gravitational potential energy = (mass of water)*(vertical distance travelled)*g

  4. Aug 20, 2008 #3
    thats a good point, i never thought of working it out with PE, that would have made it much simpler.

    Thanks for your comments they are extremely useful.

    Did u happen to look at my wattage calculation? I'm fairly confident it's correct but would be useful if someone could confirm it.

    Thanks again for your help.
  5. Aug 20, 2008 #4


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    Well your 'wattage calculation' is wrong, because (mass*velocity^2)/2 equals kinetic energy, not power.

    Power is the rate of consumption/production of energy:

    Power = energy / time

    Also, your volumetric flow rate calculation is incorrect. Volumetric flow rate equals flow velocity multiplied by the cross-sectional area of the pipe. I don't know why you've multiplied this by time. Then, mass flow rate is just volume flow rate multiplied by density; in this instance it's water so 1 litre per second is one kilogram per second.
  6. Aug 25, 2008 #5

    Do a search for "hydro power calculator". You'll find many online and several will go into the details.....frictional losses, head pressure, etc...some theoretical and some real-world.

    There's loooooots of info out there online and sometimes the hardest part of finding it is the phrasing of the search term.

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