ᐅ Decision between KfW55 and KfW40 Plus standards

Hello everyone,
we are currently planning our house and are considering whether the extra cost from KfW55 to KfW40 plus is "worth it." By worth it, I mean both the payback period and, of course, other advantages and disadvantages, such as a central ventilation system versus a decentralized one.
Our general contractor offers the following standard options (alternatives are possible in consultation):

• KfW 55 (no extra cost): aerated concrete blocks 36.5 cm (14 inches), air-to-water heat pump, underfloor heating, decentralized ventilation system
• KfW 40 plus (€25,000 surcharge): aerated concrete blocks 42.5 cm (17 inches), air-to-water heat pump, underfloor heating, central ventilation system, photovoltaic system + energy storage

My specific questions are:
1) What potential savings can be expected due to lower heating costs, electricity costs, and feed-in tariffs, assuming energy price increases, maintenance, repairs, or total failure of components are excluded? I understand that many factors play a role and this is hard to calculate, but perhaps it is possible to give at least a rough range based on comparison values and considering the following information:

• The house is located in NRW (mild climate)
• Roof orientation and pitch are almost ideal for photovoltaic
• The house has no basement and with the 42.5 cm (17 inches) blocks a living area of 190 m² (2045 sq ft)
• 2 adults and 2 small children
• 3 kWp photovoltaic system, 6.5 kWh battery (assumed)
• Indoor temperature about 23°C (73°F), bedrooms about 21°C (70°F)
• I am aware of the additional KfW funding options; however, the interest rate of the KfW loan is less favorable than that of our bank, so the advantage would be reduced to €8,150

2) What other advantages and disadvantages exist between KfW40 plus and KfW55?
3) Any further tips on how to make a good decision?
Thank you very much in advance for your assessment!
J.

Build to KfW55 standard, with a central controlled residential ventilation system, and avoid common mistakes with air-to-water heat pumps: skip the buffer tank, plan the heating surfaces carefully, and monitor everything yourself.

Install photovoltaics as large as possible and without a battery. The bigger, the greater the benefit. You can also buy photovoltaics elsewhere if the general contractor’s offer is not good.

Additional insulation is not economically worthwhile. If you plan the heating well, it’s even less so. Central controlled residential ventilation is simply better and adds pure comfort, but it is even less cost-effective than decentralized controlled ventilation.

Read up thoroughly on all these topics and your house will be significantly better than a standard KfW40 house.

The general contractor's photovoltaic system is too small. It should be larger. Otherwise, a properly sized photovoltaic system usually costs comfortably around 12,000. Then the centralized mechanical ventilation system with heat recovery will probably be about 10,000. These are two things I would have wanted personally anyway. Therefore, I would renegotiate the photovoltaic system, skip the battery storage, and instead opt for a larger photovoltaic setup. That means it would be "only" KfW 40 without the Plus standard.

Then have a chat with the energy consultant about what needs to be done to get the best subsidies.

1) About 100€ (approximately $110) per year, so it takes time.
2) Possibly a higher resale value.
3) Either you want a KfW 40 house or you don’t. There aren’t really any truly rational reasons for it.

Assign the photovoltaic system as a separate contract! You can easily exclude this trade. Just provide empty conduits from the roof to the utility room for the photovoltaic system.

What was offered to you for the photovoltaic system is a joke!

Cover the entire roof! And if you want to benefit from the $5,000 incentive for the Plus House, make the battery storage as small as possible.

Is photovoltaic included in the 25k?

For suitable photovoltaic systems, you pay at least 15k, and the battery storage already costs 5,000 (which corresponds roughly to the difference between 40 and 40+ in euros). So if you go for photovoltaic, then ALWAYS include the battery storage, which is usually included as a free extra.

You also get 10,000€ more in subsidies.

But photovoltaic and battery storage don’t really match well together. Is this the offer from the general contractor?