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Electricity Questions

  1. Jul 13, 2009 #1
    I'm generally good at physics, but electricity has always been one area that confuses the crap out of me. I'm on summer vacation right now and I wanted something to do so I was thinking of building some home made wind turbines and solar panels for generating electricity. I wanted to compare the cost of building these things and how much power they produce versus what the electric company charges, just so I can find out exactly how long I would have to run the things to see if I would ever repay the costs of them based on electricity savings.

    So if I have some solar panels that run at 18 volts and 3 amps, that's 54 watts. The power company sells energy in terms of "kilowatt-hours" so how exactly do I determine watt-hours from just watts? If it runs at 54 watts for 1 hour straight is that 54 watt hours?
  2. jcsd
  3. Jul 13, 2009 #2
    Yes, you're right.
    You'll have to let your turbine spin for 18.5 hours in order to save 1kW-hour.
    In USA the average price per kW-hour is around 9.5 cents.
    Last edited: Jul 13, 2009
  4. Jul 13, 2009 #3


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    Staff Emeritus
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    The kilowatt-hour is just a funny unit of energy (it has dimensions of [power]*[time]).

    Obviously it is bigger than a joule by a factor of 1000*3600
  5. Jul 13, 2009 #4
    Thanks for the help guys :) Costs about $140 to build the wind turbine, so to save up to repay the cost of turbine would have to run at that power rate for about 4 years, so realistically would probably take somewhere around 7 years to pay it off with energy savings. I don't think that is too bad. Considering the mortgage on the house is 30 years, by the time the house is paid off I'll also have some free energy :).
  6. Jul 13, 2009 #5


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    Staff: Mentor

    You may want to see if you can find some reliable weather stats for your area or predictions from wind turbine manufacturer's about the availability of wind power in your area. I suspect your actual utilization rate is nowhere near the 4/7 that you are using for your payback calculation.

    You may also want to compare it against other types of investments.
  7. Jul 13, 2009 #6
    Well my calculation was that it takes 18.5 hours to produce 1 kilowatt-hour. The electric company charges 7.468 cents per kilowatt-hour.

    $140 / $.07468 = 1874.67. So if I spent the money to build a tower on buying energy, i could have bought 1874.67 kilowatt-hours from the electric company.

    It takes be 18.5 hours to produce 1 kilowatt-hour.

    18.5 * 1874.67 = 34681.40 hours to produce that much energy.

    34681.40 / (24*365) = 3.96 years.

    Now since the wind isn't 100% dependable I just estimated it would take an additional 3 years to actually reach that amount. Which gave me 7 years.
  8. Jul 13, 2009 #7
    Russ was trying to tell you that you don't have to 'estimate' (guess) at this. I think you will find that the best you can do is about 20% (rather than your estimated 57%). At 20%, your payback time is 20 years, assuming the device runs without any maintenance costs...

    Now you're not going to go broke for $140, but I would look at it as something fun to work on, not really a way to save much (any) cash.
  9. Jul 13, 2009 #8
    Right, I'm mainly doing it for the learning experience and as an alternative to buying a real generator. (We need back up power in our house to keep our really large and expensive salt water fish tank alive). I posted in general engineering forum about more specifics on building it, but russ told me it was impossible, but I see plenty of sites saying otherwise...
  10. Jul 13, 2009 #9


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    Staff: Mentor

    I can assure you that it is impossible for a layperson to build such a thing from scratch. What you can do is buy the individual components of the system (generator, turbine, batteries, inverter, etc.) and assemble them, but that isn't what you said in your other thread you were trying to do. You talked about how to construct an alternator from scratch.
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