Photovoltaics initially without storage

Wonderful.... An ideal owner for a photovoltaic system with feed-in tariff.

Or how high is the allowance?

Even if you utilize the full 10kWp capacity, at most 1,000 euros in compensation (before depreciation) will remain.

However, I am not an expert in civil service law.... You shouldn’t have anything to worry about for the first 20 years.

Okay, so if we can consider feeding in after all until there are decent affordable storage systems... How does the system then have to be planned?

I have zero knowledge of civil service law. But the photovoltaic system first has to reach the point of generating taxable profit. Depreciation and other expenses related to the system counteract this. And then there is also the allowance for secondary income.

The system size is essentially determined by how much you want to spend. Then the question is how much actually fits sensibly on the roof. And there are power sizes that you should reasonably aim for, e.g. just under 7 kWp, because above 7 kWp the smart meter obligation causes additional costs. And 10 kWp is also a typical "barrier," because systems over 10 kWp are subject to the Renewable Energy Act levy, making systems between 10-13 kWp hardly profitable to operate. Above 13 kWp there is first room upwards again.

A specialized solar technician would be your contact person (not the electrician around the corner and definitely not the energy supplier).

Solarteur is the next step... I just don't want to be completely clueless at an audition... Otherwise he might sell me anything and I have no idea if it's any good... We mainly want to cover our own needs... Accordingly, not necessarily paving the entire roof...

I don't see the limit at 7 kWp as critical since smart meters will come anyway.

If there is space, go for the full 10 kWp or just under it. Feeding into the grid won't make you rich, but in my experience, it won't make you poor either. However, you will have more power in winter (which will not be sufficient with either 5 kWp or 10 kWp).

Then aim for 7 kWp. The system shouldn't be unnecessarily small, as the cost per kWp capacity decreases with larger systems, i.e., very small systems will never be profitable. Do you have a heat pump planned?

Costs per kWp capacity are about €1300 net. Larger systems are cheaper, smaller ones more expensive.