Newly built single-family house KfW55 standard - How to heat?

Is the heat pump really so electricity-intensive that a photovoltaic system pays off very quickly? Are there also statistics or figures for single-family homes based on your experiences?

Yes, the heat pump needs electricity but, when properly adjusted, has a good efficiency. In winter, it needs more energy because more heating is required and the efficiency decreases with outdoor temperatures. Still, it is a good system. Only that you generate hardly any photovoltaic electricity in winter.

Since the feed-in tariff is currently very low, I am wondering whether I should install a photovoltaic system directly on the roof at all or just leave it for now and only have the pipes laid for it.

The system no longer primarily pays off through the feed-in tariff but through self-consumption. You produce for 6-8 cents and replace a purchase from the utility of 27-33 cents. That is where the benefit lies and that’s how the photovoltaic system pays off, with or without a heat pump.

What are the costs for such a system?

A good price is probably around 1000€/kWp. But you have to find that first. For us it will be 1600€, but we have a storage included, unfortunately I don't know what that costs.

You don't have to be a prophet to say that after the next election, a green minister will be sitting in all positions that have anything to do with the environment. And they are not necessarily particularly rational. Personally, I would not install a fossil heating system anymore.

We have produced 6,200 kWh of electricity with 9.2 kWp from 17.01. until today, of which 1,500 kWh are from the period 17.01. to 01.04, i.e. in the heating period. Even in winter, quite a lot comes from the roof. So far, we have generated 55% of our electricity demand ourselves, without storage.

, with solar you usually have a buffer storage tank with 400-800l, which also needs its place.

Correct. The storage tank is not sized according to the number of people, but according to the size of the system in solar thermal applications. It must be large enough because especially in summer the water heats up very quickly and the system stagnates. This puts a heavy strain on the heat transfer fluid in the system. However, the storage tank must not be too large either, because then nothing happens during the transitional period. In addition, there must be enough space to connect and maintain everything. On top of that, there are regulations from the utility provider. I have no faith in making solar thermal look good on paper. There are cases where the necessary conditions are met through [Kontrollierte-Wohnraumlüftung]. But a professional must do the calculations.

Correct. The storage size is not determined by the number of people, but by the size of the solar thermal system.
It has to be large enough because especially in summer the water heats up very quickly and the system stagnates. This strongly stresses the heat transfer medium in the system.
But the storage must not be too large either, because then nothing happens during the transitional periods.
In addition, there must be enough space to connect and maintain everything.
There are also regulations from the supplier.

I don’t trust solar thermal for over-optimistic calculations at all. There are cases where the necessary conditions are met by controlled residential ventilation. But a professional has to do the calculations.

We wanted to have a 175l storage because, in my opinion, a 300l storage is not necessary for a family of three. Now you say it depends on the system. Could you please explain this in more detail?

175l are heated super quickly by the solar thermal system and the system stagnates. Otherwise, the water in the storage tank starts to boil and blow out. This causes the fluid in the ST system to boil, which is bad.