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Measuring electricity usage at home with solar installed

Discussion in 'Renewable Energy' started by Jules_Electronics86, Dec 19, 2016.

  1. Jules_Electronics86

    Jules_Electronics86 New Member

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    I have been reading about energy monitors available but they all seem to say they cannot distinguish between consumed electricity and generated electricity.

    Is this because they are based on a clamp current transformer sensor which can only measure current and not the direction? I have separate tails from my consumption meter - generation meter - consumer unit. If I passed the consumption meter and generation meter tails through the same clamp would they subtract from eachother? I'll probably just experiment tonight, but theoretically it should work because they are going in opposite directions?

    Thanks,

    Jules
     
  2. spec

    spec Well-Known Member Most Helpful Member

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    Hi JE,

    In theory you can measure pretty much any current, but I am not sure of the practical implications of your installation. All we can deduce is that your mains supply is 240V. 50Hz, as you live in Devon, UK. If you could post a schematic (hand sketch would do) and perhaps some images, that would help. Part numbers and links to the manufacturers data would also be a help.

    spec
     
  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Only if you are consuming and generating simultaneously. Is that possible? I was under the impression it had to be one or the other at any one time, not both.
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Theoretically, your correct. This is the basis of an RCD or GFCI. It measure the difference between Hot and neutral currents and has a winding to amplify that difference.

    The difference has a phase component, which is the direction. It may not be taken into account.
     
  6. Jules_Electronics86

    Jules_Electronics86 New Member

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  7. Jules_Electronics86

    Jules_Electronics86 New Member

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    Currently I have an optical reader on each electric meter, a clamp on the tail to the consumer unit for the house and a clamp on the generation meter tail. But I think it could be simpler than this?
     
  8. Dr_Doggy

    Dr_Doggy Well-Known Member

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    I was always under the impression that generation and consumption were separate systems where generation went straight to the grid, and consumption came straight off the grid, the hydro co then does the subtraction from the readings
     
  9. spec

    spec Well-Known Member Most Helpful Member

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    Hi JE,

    Thanks for the information in post #5; that helps a lot in understanding your installation.

    If your monitoring system takes into account the direction of current flow from the current transformer, as well as current magnitude, then your single current transformer technique of post #5 will work.

    But, in any event, I would advise connecting the current transformer as shown on the modified sketch below, which eliminates any interaction with the consumption meter.

    spec

    2016_12_19_Iss1_ETO_SOLAR_GRID_CURRENT_MONITOR_V2.jpg
     
    Last edited: Dec 20, 2016
  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    maybe this will help: https://openenergymonitor.org/emon/applications/solarpv

    Dr:
    You can do a number of things charge wise:
    1) When you generate, the meter runs backwards
    2) When you generate and use, you offset that at the retail rate but you sell back at the wholesale rate.
    3) You sell at the wholesale rate and consume at the retail rate,
     
    • Thanks Thanks x 1
  11. spec

    spec Well-Known Member Most Helpful Member

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    Hi again JE,

    Where do the outputs of the two current transformers, mentioned in post #6, connect?

    It seems to me that,
    (1) The current transformer clamped to the live tail of the generator meter will produce a current proportional to the current (I1) flowing from the generator.
    (2) The current transformer clamped to the live tail of the consumer unit will produce a current output proportional to the total current (I3) consumed by your house.
    (3) By subtracting I1 from I3 you can derive I2, the current flowing into/out of the grid supply. If I2 is negative, current will be flowing out of the grid. If I2 is zero, no current will be flowing into or out of the grid and, if I2 is positive, current will be flowing into the grid.

    So you have sufficient information to define all the currents in your installation.

    I am not sure how the two opto readers work or what values they are reading.

    spec
     
    Last edited: Dec 19, 2016
  12. Jules_Electronics86

    Jules_Electronics86 New Member

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    Thanks everyone for your input.

    I have a currentcost meter with the two optical readers and the generation current clamp attached, then an onzo energy monitor from SSE reading the actual consumption of the house. The annoying thing is trying to bring all the information together - the onzo meter can only give a realtime reading now because SSE have discontinued their website for uploading the data from it. SSE will be rolling out smart meters but they said they are not compatible with generation yet - sounds stupid to me!

    The optical readers are photo-transistors attached to the front of both electric meters and simply count the 1000/kWh pulses, I assume these readings should be exactly the same as the electric meter reading and the amount the electric company charge me.

    I wasn't able to test anything yesterday - it had gone dark by the time I got home!
     
  13. debe

    debe Active Member

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    Here in Australia if you go Solar Grid Feed the power company fits an import export meter. During sunlight solar generation if your load is under what the solar is generating, then the meter is recording exess generation. If you are consuming more than the generated solar, then it records consumed power. My system is 1.5KW &the power company charges me 31cents/Kw, & only pays me 6.8cents/Kw. It is best to use as mutch power during sun, this system is now 4yrs old. EXPORT. 21-12-16.JPG IMPORT. 21-12-16.JPG light hours.
     
  14. spec

    spec Well-Known Member Most Helpful Member

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    Hi JE,

    Would you like to build a unit that took the inputs from the two current transformers, that you mentioned in post #6, and displayed the information you require. It would not be a terribly complex or expensive unit, but it would require some electronics experience.

    If you would be interested in that approach we can post a suitable circuit.

    spec
     
  15. rumpfy

    rumpfy Active Member

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    Electricity meters are watt.hour meters and NOT amp.hour meters. They are also NOT volt.amp.hour meters.
    Accordingly, all measurements must be taken with respect to phase. Clip-on ammeters dont do the job required.

    This next bit is a rant and slightly irrelevant.
    In Australia, the whole question of so called 'smart meters' has been bogged down in political intrigue and obsession with carbon dioxide and windfarms. DEBEs' post #12 sums up the problem. Electricity costs of 31 c/kWhr are 10 times what they were 10 years ago and the problem will only get worse in Australia. Where DEBE lives in South Australia, the pollies have committed to having 40% of the states energy generated by windfarms but what they dont tell anyone is that the state is totally dependent on its next door state of Victoria to supply its shortfall from the massive brown coal deposits in Eastern Victoria. Last month, DEBEs' state of South Australia went without electricity for over 2 days I think, because the main inter connect with its life saving power source in Victoria was out of action. VERY strong winds caused the shutdown of the windfarms and there was pylon damage in a part of the state.
    I currently spend around $650 AUD on electricity per quarter, and solar is interesting but I would only do it if I had my own battery supply and used the solar for dedicated high usage items in the house like the refrigeration. 30% of my bill is a 'supply charge' which is the cost of being connected to the grid.
    Sorry for the rant, but home based solar panels are good for a holiday house where you dont use any electricity but just generate it. When the government incentives first became available, people were getting 60c/kwhr for their solar, and the proponents got killed in the rush, but of course, the scheme was short lived.
     
    • Agree Agree x 1
  16. spec

    spec Well-Known Member Most Helpful Member

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    Hi rumpfy,

    Of course you are correct, but measuring the current with current transformers was the OP's question.

    The other point is that just measuring the current and not the actual power consumed will not be that far off for a domestic installation and will certainly give the OP a good indication of how his electrical energy usage and generation compare.

    A rant indeed and totally off topic- but I fully agree with the sentiment; it is a similar situation in the UK.
    In general, the world is full of charlatans who are not only living off the backs of the 'workers' but also cause problems and reduce the standard of living by false/misguided morals and pseudo science.

    spec
     
  17. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    I'm still totally lost on what it is you are tying measure and monitor here.

    You have a meter that reads your generated solar power output and one that reads the utility power usage or feedback and a third one that reads what you yourself are actually using?
     
  18. spec

    spec Well-Known Member Most Helpful Member

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    TCM,

    I think the OP want's to see the power balance at any one time, rather than the accumulated energy balance, which the meter readings would give. That's my understanding anyway.:)

    spec
     
  19. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    That's what I am guessing but I have no clue as to the why part. I have played around with co generation for years and I have never felt that I needed to know anything about my direction of power flow beyond the main reference wattmeter spinning one way or the other.
     
  20. Jules_Electronics86

    Jules_Electronics86 New Member

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    The original question was asking if a single current transformer could measure the consumption (which should be the same as what I get charged for) for my house without the generation affecting it. The three meters I have are annoying to read and I can't easily download the data. I've tried using the online meniscus application for the currentcost meter but it's not very good. I will probably try the labview application again because at least then I can mess with things I want to do differently.

    I'm trying to get back into programming in C as it's been a while, so bought a MPlab xpress eval board, previously used pickit1/2. My hope is to be able to monitor the excess being generated and use it to charge car batteries. Ultimately monitor the charging and discharging and even make some of my appliances 'smarter' using a micro controller.
     
  21. spec

    spec Well-Known Member Most Helpful Member

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    Hi JE,

    I'm getting like a parrot saying this, but an Arduino, or Raspberry Pi are made just for you and they cost peanuts.

    You could have pretty much any combination of input and display, especially as you are a C man.:cool:

    spec
     

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