Air conditioning: What is the royal way

210 are already a very low assumption

We have decided against it... A will move out anyway at some point and B... if it is so hot at night that air conditioning is necessary, the children are allowed to sleep in our bedroom or in the [Wz]

210 are indeed a very low assumption

From our experience, I can report that the 210 euros are calculated very well. That would mean that the system runs 3 hours a day at 40% (e.g., living room and 1 child's room). We have been living here for 9 months now and have consumed about 3200 kWh. Since the controlled residential ventilation also consumes electricity, there is hardly any room left for high air conditioning consumption.

Same here...we actually pay about four times as much

We have been living in it for 9 months now. And have consumed about 3200 kWh.

Then the first full year's consumption remains to be seen. The mentioned 9 months mostly include cold to moderately warm months. The hot three are just coming now.

We thought about air conditioning but postponed it. That is definitely a luxury for us. Shading works as well, and for 3-4 days of extreme temperatures I won't spend the money.

With us, the retrofit would now cost 5,000 euros including installation for a 7.5 kW unit with 3 "Premium" indoor units from Mitsubishi. I calculate the electricity costs differently and also come to lower values. It must also be taken into account that the utilization factor is very low when the heat input from outside is kept small. In any case, I would make the investment immediately if I notice that it becomes necessary. For a good night's sleep, 10 euros can also be spent. So far, however, we keep our house very pleasantly cool at night by simply ventilating, and during the day we have not had more than 24 degrees on the ground floor and 25 degrees on the upper floor. I myself am surprised how little the upper floor heats up, maybe the Harz under-roof makes a difference there.