Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

KW Volts Amp Confusion, Please Help

  1. Jan 18, 2010 #1
    KW, Volts, Amp always confuse me. I have a project of 10kw generator, how do you measure that it is 10kw at first instance. for example if the output is 1200 volts, can it become 10kw or be a 10kw generator etc. What is the power requirement standard of a home ? how to fit the 1200 volt generator of 10kw suitable for house use ? please help.
  2. jcsd
  3. Jan 18, 2010 #2


    User Avatar
    Science Advisor

    You would get the voltage and KVA rating from a maker's pad on the side of the generator.

    To get this 1200 volts to 120 volts you would have to use a 1200 volt to 120 volt transformer.

    The power supply power out depends on the power factor of the load.

    If the transformer is rated at 10 KVA and you have a perfectly resistive load, the output power of the generator would be 10 KW. There would be losses in the stepdown transformer, but these are quite efficient, maybe 95 %.

    The generator can only supply 10 KVA / 1200 volts or 8.33 amps maximum. If the load is less than this, of course it supplies less than this.
    At the house end of the transformer you might get 95 % of (83.33 amps at 120 Volts). That is still a lot of power, but it depends on the appliances used in the house whether it is enough.

    If the load is reactive and has a poor power factor, the generator might still provide 10 KVA but some of this will not be available as power.
    Loads like motors and fluorescent lights can have poor power factors if they are not corrected.

    There is an excellent article on power factor on Wikipedia:
    The graphs show the effects of poor power factor very well.
  4. Jan 18, 2010 #3


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    The point about the VA rating of a power supply device is that there is a limit to the current it can supply (wires get hot etc.) and a limit to the Voltage it can handle. If your load is very reactive, the maximum current it will draw will not be in phase with the maximum Volts across it (or it may not even be proportional). This means that the actual power it uses will not be equal to V(average) times I(average) but its maximum, separate, voltage and current requirements will add up to a higher overall V and I demand on the supply.
    The Power Factor will tell you how much less actual power you will actually get (or how much bigger the demand on the supply will be) but it assumes that the load is linear. Power control circuits, which switch within the mains cycle, will cause demands on the supply which the Power Factor figure may not describe adequately. When PF was first used, there were no such devices.
    If you have your own private generator you may find it is easier just to make sure your load is kept well within the spec of your supply. On the brighter side, the actual fuel consumed relates pretty closely to the energy you actually use - there will be very little extra loss due to a bit of excess current unless you have an unusual set of loads.
  5. Jan 18, 2010 #4
    Typically, VA is simply Volts x Amps and Watts is Volts x Amps x PF, where PF is the power factor. This is basically the percentage of power that is actually being used. Often, this value runs at around 90%.
  6. Jan 18, 2010 #5


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    But only when the load is linear and the waveform is a true sinusoid. It's near enough for most purposes or, at least, it has been. Things may be different when power sources and loads are not like the conventional ones.

    Just think how really efficient high-power DC voltage-changing systems could change things. High voltage transmission would be less lossy and there would be no problems of synchronising generating sets.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook