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Metering net solar exports

Discussion in 'Electrics UK' started by cjcoffin, 27 Apr 2021.

  1. cjcoffin

    cjcoffin

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    We have a standard 3.6kW solar setup installed in 2011, it has a Total Generation Meter fitted inside between the inverter and the CU. Outside in the meter cupboard there is an old two-rate meter for the E7 supply.

    Neither of the above measures the amount exported (and it is not possible to infer it from their readings because of the unknown internal consumption of solar power).

    So I want to add another meter to measure actual export (and ideally with an extra register for total import, which will enable me to check the DNO's meter. I do not want to change this for a smart meter.)

    What should I buy and what is the recommended way of fitting it?

    TIA, Chris
     
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  3. flameport

    flameport

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  4. cjcoffin

    cjcoffin

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    If I get the DNO to fit an isolator, which I would like for other reasons, why can't I then fit my own meter between it and the CU?

    Or is there a solution of the clamp-on type? I would have gone for Owl but I see the s/w has poor reviews, are there reliable alternatives?
     
  5. ericmark

    ericmark

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    There are units [​IMG]that claim to measure whole house which simply clip on, I was given one by Scottish power, however there are two problems, one it does not show direction and two it does not show power factor.

    Also the one shown only goes to 7200 VA and 7.2 kW is hardly enough for a house.

    I would assume the meter measures voltage either side of a very low resistance to detect direction of power, I have never been where power can flow either way, so never looked at how the meter works out direction.

    I proper electric meter measures watts, but the cheap meters measure amps and assume volts is 230 volt. It depends on if you can actually clip onto a cable with supplies everything but solar panels. In broad terms energy used by whole house minus energy produced by panel should equal energy charged for, but as far as I am aware the early solar panels had no real way to show how much was exported, so the house holder was credited with what it should produce, so we had devices to ensure we used the power and did not export it, heating the immersion heater for example.

    I was lead to understand the whole idea of the smart meter roll out was so meter could measure direction of flow? As to if as house holder it would be worth having a smart meter I really don't know, some of the early solar panels basically gave way money which was taken from everyone else.
     
  6. flameport

    flameport

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    The fact you are asking how to fit it indicates you are not competent to do so.

    It's necessary for the device to have connections for both current and voltage, there is no clamp on solution as those only measure current and will be unable to determine whether power is being imported or exported.
     
  7. cjcoffin

    cjcoffin

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    Thanks ericmark, I have also been looking at the efergy engage product range but although it can have up to 5 current transformers it is not clear if it will do what I want.

    I know how to fit it, I am a Chartered Electrical Engineer. I was asking why you originally said I can't fit it myself.


    Yes but assuming there is a mains-powered display and/or wireless receiver or internet hub somewhere it can measure the mains voltage at some point in the system
     
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  8. BS3036

    BS3036

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    In my system, I put a Henley block after the meter, and both the solar generation and the house demand are connected directly to that. Two clamp probes are fitted one to generated side and one to import side. The Wattson displays (amongst other options) the difference between the two probes, thus import or export at any given time. Software was available to keep records. Unfortunately the Wattson is no longer available, but other solutions may exist.
     
  9. JohnD

    JohnD

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    the voltage in my house is always around 240v. there is no need to measure it to discover that.

    my meter turns on a green LED when it is exporting. I suppose you could add a sensor to detect it. It only meters imported electricity. So I suppose if I bought my own and connected it backwards, it would only measure exported electricity. I'm told that the old spinning disk meters ran backward while exporting (though stepped cogwheels might not work) so I suppose you could buy an old one and know the net imports.

    I use Owl monitors, which record onto my PC. They have clamp pickups and don't know if they are measuring import or export, but I can tell by looking out of the windows and knowing what appliances are in use.
     
    Last edited: 27 Apr 2021
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  11. ericmark

    ericmark

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    I have never stripped one of the old meters, but I think unlikely they can detect direction. A simple op-amp can detect direction, not hard to do, but until micro generation we did not need to know.
     
  12. StephenStephen

    StephenStephen

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    I wonder about a harvi and hub from myenergi?
     
  13. cjcoffin

    cjcoffin

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    Thanks, looks promising but I can't confirm from their website what the actual functionality is, it all seems to be guff about the downloadable updates and 3-yr warranty! Anyone else tried one?
     
  14. plugwash

    plugwash

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    However you need the actual voltage and current waveforms to calculate the net power transfer. Just knowing the approximate magnitude of the voltage waveform is not enough.
     
  15. JohnD

    JohnD

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    Maybe.

    What do you "need" it for, and what will you do with the information?
     
  16. plugwash

    plugwash

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    Fundamentally to calculate the net power transfer you multiply the voltage waveform by the current waveform to get the power waveform. Then you take an average of the power waveform.

    You could make an educated guess that the voltage waveform is a sinewave, but that still doesn't tell you much because you don't know it's phase relationship to the current waveform.

    Compare the means of

    sin(θ)*sin(θ) -> 0.5 -> net power transfer in one direction
    sin(θ)*sin(θ+½π) -> 0 -> no net power transfer
    sin(θ)*sin(θ+π) -> -0.5 ->net power transfer in the opposite direction
    sin(θ)*sin(θ+¾π) -> 0 -> no net power transfer
     
  17. StephenStephen

    StephenStephen

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    It's a system which is intended to manage their solar current diverter (eddi) and car charger (zappi) and home battery, but I'm thinking that just a hub (connects to internet) and harvi (measures electricity) could do what you want.
    I've been using it for a couple of weeks, and it seems to work well.
    Whether it will do what you need, or it's accurate enough - I don't know at this point
    There's a relatively new forum: https://myenergi.info/
    ...and myenergi respond within a day or two to emails
     
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