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

KWh to KVArh conversion

  1. Jun 29, 2015 #1
    H Everyone,

    I have a question possibly someone can help with.

    I provide basic remote GSM monitoring for a client of mine which we count pulses from an energy meter to count KWh's. I am able to provide basic energy costs based on the tariffs provided by the utility company. To provide a more accurate cost of energy I need to be able to work out the KVArh.

    Here are the known inputs and outputs,

    3 phase
    380 volts
    measured KWh from energy meter pulsed outputs.

    Question: is it possible to work out the KVArh from the KWh readings?

    your assistance would be greatly appreciated.

  2. jcsd
  3. Jun 29, 2015 #2
    Based on the info you have provided I would say no. You will need at least one more piece of data - depending on how accurate you want the result. Since you are using pulse data I am assuming your data is very limited.
  4. Jun 29, 2015 #3
    Hi Windadct, is there a key input that would assist me in extrapolating KVArh from KWh that I am missing?

  5. Jun 29, 2015 #4
    Not to joke but KVARH - if you have amps for example, you could do the calculation ASSUMING constant Voltage - i.e. less accurate. KVWH meters look and behave exactly the same as KVARH - one measures REAL power = Watts = Volts*Amps REAL, one measures REACTIVE POWER = VAR ( Volts * Amps Reactive) .. the REAL issue confuses a lot of people.
    There are relatively low cost methods to measure the key parameters - but if the measurement is not there, you can not figure this out based on the data you listed above. - In short a simple pulse stream will not get the data you need.
  6. Jun 29, 2015 #5
    A number of data could be used depending. Power factor would be useful if it remained more or less constant (or you were able to sample it as well.). What he's using the electricity for could also provide a crude estimate. Power company charges might help as well. (They sometimes charge extra for reactive power.)
  7. Jun 30, 2015 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    known inputs?
    3 phase - what do you mean by that ? Do you have three measurements of phase current ?
    380 volts -
    what do you mean by that ? Do you have three measurements of phase voltage ?

    If all you have is pulses from KWH meters
    so far as your computer knows KVARs are not even measured.

    Inquire of your client if he has KVAR meters that you could monitor. Sometimes they're called Q-meters.
    Last edited: Jun 30, 2015
  8. Jun 30, 2015 #7
    If all you're getting info from are KWh meters then that's what you've got. If you want KVAh, of any sort, I would think you'd need to get something from a meter that's measuring KVAR's...if you had that, then you could derive KVA pretty easily.

    NOTE: I'm no expert on metering, this is just from what I catch around the workplace...
  9. Jun 30, 2015 #8


    Staff: Mentor

    I think you are suffering from a misconception. Watts and VARs are orthogonal in the same way that an X axis and Y axis are orthogonal, or latitude and longitude are orthogonal. Orthogonality means that one can not infer one from the other, or "extrapolate" on from the other.
  10. Jun 30, 2015 #9
    While you are right from an engineering perspective, this seems more of an accounting issue.

    The real question is, how accurate does this need to be. Since it's only sampled, I'm assuming not that much.
  11. Jun 30, 2015 #10


    Staff: Mentor

    To measure, there are two approaches.

    1. Measure instantaneous V and I, sampling period 1 ms or less. VA is RMS(V)*RMS(I). W= RMS(V(t)*I(t)), VAR= SQRT(VA^2-W^2), where RMS() is root-mean-square of an entire cycle's values. Accumulate the values over time to get kWh and kVARh. If your measurements are going into a microprocessor, the math is not difficult.

    2. Use a kWh meter plus a separate kVARh meter. Pulsed or continuous.
  12. Jun 30, 2015 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    KWH they'll want to great precision because that's revenue.
    KVARh they'd probably accept with less precision because it's an adjustment, but significantly as Anorlunda pointed out KVAR will need to be integrated .
    Just to average sampled instantaneous power factor readings would be a mistake of arithmetic

    KWH meters perform a real time integration to report energy from a power measurement.
    I'm accustomed to the old electromechanical type with a rotating disk. They are remarkably accurate especially considering how long ago they were invented, late 1800's. Ours gave a small number of contact closures per turn of the disk (two or four or six i forget ).
    Each turn of the disk represents so many watt-hours and that is indicated on the meter face by Kh . So one adds so many kwh to his running total upon receipt of a pulse.


    I suppose if you had a measure of instantaneous power factor or instantaneous KVARS you could integrate them in software.

    Page 169 of this PDF begins a detailed explanation of the disk KWH meter.

    old jim
    Last edited: Jun 30, 2015
  13. Jun 30, 2015 #12


    User Avatar
    Science Advisor
    Gold Member

    You could purchase a Watt/Var transducer for real time measurements and integrate as needed. Here's one from Omega.

  14. Jun 30, 2015 #13
    In your original statement you gave the Knowns (3 ph and V) and the Output ( pulses) -- but reading this again I have to ask.... Is the utility billing for VARs and how are they measuring them? If the cost of the energy is including some VAR measurement - then there should be a meter for them, if oyu have acces to the Pule outout of the utilities WH Meter - then you need access to their VAR Meter.
    If the customer has installed his own meter to be able to cross check the Utility - and he only has a WH meter - then you can not get a VAR number accurate enough for revenue. If the Customer's self-installed WH meter is a socket type it can be replaced directly with a higher function meter. EI 1262.

    Still based on the data in your original post - you can not figure out the VARs without adding metering or finding a new data point.
  15. Jul 1, 2015 #14


    User Avatar
    Science Advisor
    Gold Member

    Absolutely. For billing, a transducer isn't acceptable.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook