My total calculation resulted in an energy demand of 5000 kWh/year for the "KfW 40" house with 81 sqm living area. It is possible that I miscalculated somewhere.
You cannot calculate it like that or some assumptions are too optimistic.
For the transmission losses, always use the outer surface area.
Additionally, add a flat 0.05 W/m2K for thermal bridges on the wall area.
For windows, only 0.6 for the glass (Uw-value). With the frame rather 0.8-0.9 (Uf-value).
For the total calculation do not use average temperature and days.
Find the heating degree days for 22° indoor temperature and 15° heating limit temperature for your region.
You then apply these also to the ventilation losses and calculate with an air exchange rate for the total volume of 0.4/h-0.5/h.
With heat recovery, you can deduct 50% from that. The system may have 90% efficiency, but you also have leaks.
I have just calculated that here for another house. I just can’t find it right now.