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What battery inverter to choose

I am interesting in upgrading my existing PV System with an DIY Powerwall to increase my own Solar Energy during the night.
My existing Setup is:
2 String PV generator 15 and 14 modules, 5960kWp total.
The existing inverter is a 5500W 3 phase AC inverter 230/400V( I am in Europe/Germany)
There is a 2 way Energymeter that mesures the Grid Buy and the Grid Sold.
I Get aroud 30% self usage of the PV Generator per Year.

Now i want to add a Battery Inverter (charger/inverer) that charges a DIY Battery from 18650 Cells and feeds the house during night to increase the usage from the Solar Energy.

Is there a product (230V single phase batteiy inverter/charger) that can be added on the AC side after the existing Inverter  an that can handle a 10P or 14P Battery setup.
I think it also needs a smart Energy Meter to control the House/PV and Grid Power.

I dont need to be a 5000W System, i think 1000W or 1500 W on single phase is more than enough for the night (fridge, TV,...)

note, i dont want to disconnect the PV strings from the existing Inverter, because it logs the Data on a Web based site, i need the records for the Tax rating at the end of the year.
So the Update needs to be an the AC side between the existing inverter and the Powermeter from the Grid company

can you please help me

Thank you alot
There is no such system that saves you money to be honest. Not in the way you ask and thats because:

solar -> Inverter -> AC -> Charger -> inverter

As you can see you will be having a huge loss due to converting it back and forth between AC and DC.

Handle 10p/14p what? Do you talk about 10s aka 36v or 14s aka 48v system? Yes they exist but see above. 14s is the most economical

Your biggest bet would be to get an offgrid inverter that you move over solar panels to and use for charging and running your "static" loads. And then closer to winter you just shut this system off and run it as is.

Also note that the cheaper gear you buy the more losses you will have and if im not mistaken the electricity isnt that expensive so to overcome that you need to look into idle load and losses. As an example my 10kW hybrid inverter need 7kWh per day to just being turned on!!!! Thats alot...
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