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Member's Forum What is likely to happen, if anything, if I try and export to the grid and import from the grid simultaneously?
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What is likely to happen, if anything, if I try and export to the grid and import from the grid simultaneously?

Andrew J
Andrew J 3 months ago

I have this scenario:  I want to export from a house battery to the grid during the period 23:30 to 01:00 at 3.6kW (until battery is empty.)  At the same time, I have my car plugged in charging at 7kW.  Would that have the net effect of pulling 3.4kW from the grid?  Is anything likely to fail?

As a basic summary, I can charge my house battery up at 7p from 23:30 to 05:30 and discharge it to the house during the rest of the day, thus I pay only 7p for electricity used.  I don't fully discharge the battery during this period so I can sell the remaining charge back to the grid at 15p before fully charging up again ready for the day.  However, I also need to charge my car up which I also do 23:30 to 05:30 - Note because of the tariff I am on I have NO control over when the charging actually occurs so I cannot configure it to avoid the time period where the house battery is discharging.  Re-configuring the battery to not discharge when I plug the car in is a laborious process which I want to avoid.  I only actually charge the car once or twice a week.

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  • kmikemoo
    kmikemoo 3 months ago in reply to Andrew J +3
    As long as your grid tie point is behind the meter, you are correct that you will consume what you feed into the system and just draw the balance. If your grid tie point is separate or ahead of the…
  • Andrew J
    Andrew J 3 months ago +1
    To close this off, everything worked fine - car charged up, battery discharged and then charged backup. No fiery deaths occurred. What I’d forgotten about was the 8kW inverter - it can discharge up to…
  • Andrew J
    Andrew J 3 months ago in reply to obones +1
    Unfortunately the battery price wasn't broken out in my invoice, however looking back at emails, an 8kW pack cost £2350 and 5kW £1950. This is ex-vat and doesn't include installation or other ancillary…
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  • robogary
    robogary 3 months ago

    Theoretically - yes. Nothing should fail as long as both are wired and protected to handle the currents as they occur individually. Grid battery storage does the same thing. Don't think the battery gets "empty" tho.  

    Your electricity cost must be absolutely outrageous if you save money using battery storage and only connecting utility grid at night , unless you omitted having daytime solar charging system. Your car does have a plug, so there is control when you charge it up. 

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  • robogary
    robogary 3 months ago

    Theoretically - yes. Nothing should fail as long as both are wired and protected to handle the currents as they occur individually. Grid battery storage does the same thing. Don't think the battery gets "empty" tho.  

    Your electricity cost must be absolutely outrageous if you save money using battery storage and only connecting utility grid at night , unless you omitted having daytime solar charging system. Your car does have a plug, so there is control when you charge it up. 

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  • Andrew J
    Andrew J 3 months ago in reply to robogary

    As I say, I don’t have control over when the car charges, it is a schedule set by my utility company based on some algorithm they have otherwise I would just schedule accordingly. The 16kW house battery is just part of the overall system. 

    But I’m interested in my question: I think the outcome will be either a net pull from the grid, or the battery just won’t discharge. 

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  • kmikemoo
    kmikemoo 3 months ago in reply to Andrew J

    As long as your grid tie point is behind the meter, you are correct that you will consume what you feed into the system and just draw the balance.

    image

    If your grid tie point is separate or ahead of the meter, your inverter needs to be capable of Export - which based upon your original post, I read it to be true.

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  • robogary
    robogary 3 months ago in reply to kmikemoo

    good point, I'd assumed the wattmeter is the point of entry from the grid. I also assumed the inverter is regenerative, but that may not be a supported feature  :-)  

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