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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

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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  • 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 that rate although if the discharge is to the grid it limits that to 3.68kW.  Last night with the car pulling 7kW, this initially came from the battery which was also then able to send 1kW back to the grid.  Once the battery had discharged all power came from the grid.

    The underlying complications are that:

    • I don’t control when the car charges.  I plug it in, my utility company recognises that and sets a charging schedule based on my desired SoC, ready-by time and expected grid load.  This can change as time goes on and grid load changes.
    • The battery configuration is awkward.  I can’t amend a configuration, it has to be deleted and then re-created, so it’s a pain to reconfigure it to not export to the grid whilst the car is plugged in, then change it back again.

    I’m hoping to get more control over this with Home Automation but I’m away off that at the moment, so the fact I can just leave the battery configured to export then recharge makes life easier even if I lose out on the pennies.

  • in reply to Andrew J

    Hi Andrew, it's not physically possible to import and export simultaneously so, as you observed, the battery discharges to the car first and then the grid takes over.

    Charging the car from your battery is not the best use of the energy (average round trip efficiency for battery charge and discharge will be about 87%). It's not a big issue, but you should be able to stop this happening with a bit of automation. 

    In that regard I highly recommend an app called Netzero. I use this to stop my Powerwall from charging the car when Octopus schedule a charge outside the cheap rate window.

    Best of luck 

    David

  • in reply to Dunstan456

    The charger connection is being rearranged next week so the battery never sees it which will help.  Do you have a link to that app; Net Zero, in any form, is a popular search at the moment,

  • in reply to Dunstan456

     Thanks for the suggestion David, I've had a chance to take a look.  I was expecting a Home Automation integration for some reason not a standalone app - which would have been great for my purposes!  However, I have a Siggen Energy system not a Tesla Powerwall so it won't work for me.  Looks useful though. I have my installer back tomorrow and can get him to set up the ModBus feature on the Inverter so I should be able to connect to it from HA and can then see what I can/cannot do programmatically.

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  • in reply to Dunstan456

     Thanks for the suggestion David, I've had a chance to take a look.  I was expecting a Home Automation integration for some reason not a standalone app - which would have been great for my purposes!  However, I have a Siggen Energy system not a Tesla Powerwall so it won't work for me.  Looks useful though. I have my installer back tomorrow and can get him to set up the ModBus feature on the Inverter so I should be able to connect to it from HA and can then see what I can/cannot do programmatically.

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