Heating decision: air-to-air or air-to-water heat pump

Aside from the fact that it does not necessarily have to be company A or B, was the air-to-air heat pump used in a KFW55 house and the air-to-water heat pump in a "climate-friendly residential building – with QNG," i.e., KFW 40+?

In combination, I would not choose the air-to-air heat pump. If air-to-air heat pump, then at least KfW40. I would also find that very elegant for a passive house during the coldest 10 days of the year. Whether you then certify it sustainably and add a battery is another question. Basically, you should not expect too much from photovoltaics in winter either. Short days, low sun...

Basically, you shouldn’t expect too much from photovoltaics in winter either. Short days, low sun...

Not even with a 'full occupancy' of the roof? I would have thought that would be enough. So actually, we want to go for kfw40+ with photovoltaics + battery for the subsidy.

'Full occupancy' of the roof

That is already the prerequisite for a productive winter, if there is no snow on it. Self-sufficiency in summer is something anyone can do.

The blanket statement "roof full" from the relevant photovoltaic groups is not wrong but also only half the truth. In winter, the sun simply stands differently, and for example, I could of course still completely fill my east-west gable roof, but in winter, that would do me little good because of the angle. You can simulate something like that for yourself. On the other hand, if I mount significantly less area at the right angle on the south side, I get significantly more.

In general, wishes like "absolutely one hundred percent self-sufficiency on all days of the year" quickly become very expensive. Sure, you can cover infinite space and build infinite storage to still comfortably run the sauna even during a week under -25°C – but then it is knee-deep in hobbyism. The other extreme are the "roof full and feed-in" people who still have old contracts with high remuneration. That pays off great, but if (as happened to us recently) a lightning strike fries the distribution box, it quickly gets very cold in the house.

So it is always an individual balance between objective benefit, hobbyism (I do not mean that negatively; my system has a lot "because I want it"), and the available budget.

Full agreement.
The big advantage of a full roof, however, is seen in cloudy weather or fog.
It is always calculated as if the sun shines 365 days a year.

With a closed cloud cover, the angle and the direction are pretty much irrelevant.
That means double area = double yield. At least almost.

For profitability, the angle must of course still fit reasonably well in sunlight, that goes without saying.
But there are plenty of programs that simulate that for you.

Where does filling the roof not make sense?

The price usually always decreases with larger systems.
You only need one scaffold, (usually) only one inverter, one installation, etc.
Therefore, it is very, very rarely sensible to stop halfway!