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DO YOU KNOW? Inverter conversion formula? #2

Discussion in 'Electronic Basics' started by Dave, Jun 7, 2004.

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  1. Dave

    Dave Guest

    Just curious if there is a set formula for calculating how many amps a
    12 volt car inverter draw from battery given the load of an AC device?
    Was my original question, had a few more...

    Got this as an answer (thanks Soren):

    Input current = output power / (input voltage * inverter efficiency)

    Input 12V_DC
    Output Load 50VA ("RMS")
    Inverter effiency 70%

    Input current = 50VA / (12 * 0.7) = ~6A

    Does this equation apply for an 120VAC device plugged into the 12V_DC

    Is the equation different for resistive and inductive loads?

    Being new to the terminology, is VA truly equal to Watts?

    So lets say I have a TV plugged into my inverter: needs 120VAC of
    course and draws who knows, 1.2 amps. Would the above equation apply?

  2. No. Watts equals VA only at Unity.

    There is this thing called the power triangle. Watts is the x axis,
    VArs is the Y axis, and VA is the vector that results. Simple
    trigonometry can figure out any one of them.

    Watts, in fact, equals VA times power factor. (Also known as Cos theta.)

    So, if you are drawing 5 amps at 120V, you are consuming 600VA of power.
    If your power factor is 0.5, then watts equals 300W. This is where the
    hydro utility gets upset, as you are getting free power.

    There is also 520 VArs in there too. That 3+4=5 rule kicks in to
    generate that component.
  3. K Williams

    K Williams Guest

    At unity power-factor.
    Cos(theta) works only for sine waves. A more general formula is PF
    = watts/VA.
    No you are not getting free power. They're generating power that is
    not being billed, but you cannot do work with VARs.
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