Obtain a photovoltaic system through prefab house builders?

First, spend some time getting familiar with photovoltaic matters. It's the most sensible thing anyway since you plan everything roughly yourself. Then you'll know your way around it and will be in control of your system.

When you know everything, order the modules, some rails, and the cables. When the roofer comes, I would have them installed at the same time. Ideally, have a conduit laid from the house to the garage. Install the inverter and storage there, and when the electrician comes, have them make the connections in the meter cabinet. Ideally, they will also handle the registration with the grid operator.

If the craftsmen are there anyway, they will do it as well. Of course, for payment. You should be able to realize the photovoltaic system for about half or even significantly less.

Choose a Huawei system. Affordable, very good quality, good support, and no problems with registration since all certificates are available.
Modules don’t really matter. Trina or other cheap Chinese products are practically given away these days. Inverter and storage can still be purchased without VAT.

At least demand, consumption behavior, roof orientation, etc. were not taken into account. Probably only the roof area. Although opinions differ on the dimensioning... 10 kWp is a harmless size

No no, you misunderstood that. He’s not interested in your demand, but simply in the naive calculation that you should put as much photovoltaic as possible on the roof because it then becomes “cheaper” per kWp. Who doesn’t buy 100 screws for 11.99 EUR when he only needs one, which then of course costs 1 EUR. Per piece the screw is cheaper, but you still spend 10.99 EUR more for 99 screws you don’t even need.

Especially since the fun is financed at 4%. If you have the money, photovoltaic can pay off abundantly. Financed at 4% over 20 years... no. Especially not if you put 20 kWp on the roof of your single-family house and then feed 80-90% into the grid as a mild gift...

I find it a bit expensive to buy a 10 kWp system with a 9.6 kWh battery for 26,000 euros including installation. What do you think?

Last year I had a system with the same parameters installed on the roof for 18,000 euros. Currently, depending on the exact manufacturers/components you want, it’s a few thousand euros cheaper.

Make sure to have the house builder lay a conduit. Get quotes for the system and have it properly dimensioned.

Rough calculation: In principle, a storage system only makes sense if it does not cost more than €300 per kWh of storage capacity. This is a realistic rule of thumb for the average photovoltaic builder/user. Otherwise, the storage electricity simply becomes too expensive. So 10 kWp for €1,100 per kWp + 10 kWh storage for €3,000 = system price of €14,000 (which is a very cheap price), realistically more like €16,000-18,000 on the market.

: PM apparently didn’t work. Check out PVAndyE. They will help you there. You wanted a puzzle anyway.