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Discharging battery to grid whilst charging EV

 
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 AndrewJ
(@andrewj)
Trusted Member Member
Joined: 4 months ago
Posts: 32
Topic starter 14/05/2025 10:53 am  

What would be the outcome in this situation?

I'm on Octopus Intelligent Go so smart charging can set a schedule at any time period without my control.  I have 16kW of home battery storage (SigGen) which powers my house during the day and is recharged during the cheap 23:30 - 05:30 period.  I want to be able to discharge the battery at 23:30 to the grid to be paid 15p/kW and then charge it back up again at 7p.  

The only strategy I have at the moment is to configure the SigGen batteries to discharge 23:30 to 01:00 and then charge 01:00 to 05:30.  If I plug the car in, I reconfigure the SigGen batteries to only charge between 23:30 and 05:30.  Then, in the morning, change the configuration back to my discharge/charge settings.  Problem is it isn't possible to edit settings, I have to, essentially, delete the configuration then set it back up again.

It's not that difficult but it is a faff and is subject to me remembering!  Given I charge the car maybe once or twice a week, what would happen if I leave the configuration of the battery alone and the scenario arises that the smart charge schedule covers 23:30 to 01:00 and thus pulls from the grid (7kW) whilst the battery is trying to discharge to the grid (3.6kW)?  I don't want to break anything.

If it's not a problem then I'll just forego the 15p payment on the evenings where the car is charging.  I'm hoping to get Home Automation up and running which I think would sort this but I'm finding it a bit confusing to say the least so it will take me some time.


   
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Majordennisbloodnok
 Majordennisbloodnok
(@majordennisbloodnok)
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Joined: 3 years ago
Posts: 791
14/05/2025 1:12 pm  

I'm going to qualify what I say right at the start by saying I'm no expert so if anyone contradicts me later they may well be right.

That said, I don't see how you could break anything but things might not be quite what you expect. Eventually, you only have one cable from the grid to your house and your import or export is simply a measurement of the net flow of electricity in one direction or the other. Therefore, if you charge your car at 7kW at the same time you're telling your battery to discharge at 3.6kW then the car will charge from your home battery and will pull extra from the grid to get up to the full 7kW. That's not ideal since you're not going to be paid for the energy being discharged from the battery and you're also wearing your home battery unnecessarily.

You should also bear in mind the likely lifetime of your battery vs its initial cost so you can get an idea of how much a charge/discharge cycle costs. You may well find that the 8p difference between export price and cheap import price isn't as much after all. I've been through it and found playing the "import now to export later" game gives only relatively low payback.

All that said, you're quite right that a bit of home automation is the way to go to get much better control (along with keeping an open mind on tariffs too, of course). Assuming you're talking specifically about Home Assistant, I'm sure there's plenty we could do to help reduce the confusion so shout if you want help.

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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