Air heat pump or use gas and solar?

Unfortunately, I cannot calculate the heating load because I am missing the relevant data from the architect. We are still at the beginning of the planning phase. So it is not so easy to say based on a single-family house KFW 100 according to the 2016 Energy Saving Ordinance whether the operating costs are cheaper with a gas boiler than with an air-to-water heat pump or how high they are approximately per month?

Please do not take the question badly, as unfortunately I do not know much about these things.

But since I work in the energy sector, I estimate (this is what the planning and assessments look like) that in the coming years the electricity price will rise due to the Renewable Energy Sources Act surcharge and grid fees (power line from north to south) and the gas prices will remain rather constant.

My general contractor created an economic efficiency calculation for an LWZ 304 for me at Stiebel Eltron (Stiebel offers a corresponding tool for commercial users, specifically for their own products; to what extent this is beyond any objectivity, you have to evaluate for yourself; the heating load check and comparison of independent offers are still pending in our case).

Kfw55
heated area 140m² (basement, ground floor, attic)
average room height 2.50m
4 people

Building heating load 3.5KW
Standard outside temperature -14°
Heating limit temperature 10° (is this the bivalence point?)
Supply temperature 35°
Coverage rate 98%

LWZ304
Monoenergetic
Bivalence point -5°
Underfloor heating

Domestic water normal demand

Cost LWZ including ventilation system without installation: €19,057 (of which €14,500 for the LWZ itself).

The following economic efficiency calculation (copied from a table, unformatted):

Consumption-related costs Natural gas Heat pump
Energy price heat production [Cent/kWh] 7.50 19.00
Energy price auxiliary energy [Cent/kWh] 25.00 25.00
Base price per year [€] 205.00 67.00
Energy costs of the system [€] 833.00 539.00
Energy costs/living area [€/m²] 5.95 3.85

Operating costs Natural gas Heat pump
Maintenance and repairs [€/year] 25.00 10.00
Cleaning of the flue gas system [€/year] 75.00 0.00
Operating costs of the system [€] 100.00 10.00

Total costs Natural gas Heat pump
Consumption- and operating costs [€/year] 933.00 549.00
Investment costs [€] 19,700.00 25,657.00
Capital-related costs [€/a] 1,450.00 1,888.00
Maintenance costs [€/a] 385.00 95.00
Total costs of the system [€/a] 2,768.00 2,532.00
Total system costs/living area [€/m²*a] 19.77 18.09

Amortization Your data
Observation period [years] 20
Amortization [years] 14

Boundary conditions Your data
Observation period 20
Interest rate 4
Price increase natural gas %/year 5
Price increase heat pump electricity %/year 2

Criticism points:
- I assume that installation costs are not included here?
- no exact information about the GAS system
- Price increase for natural gas was estimated at 5% and electricity at 2%
- Can the bivalence point be set freely? Does -5° make sense here or is this also a manipulated calculation?
- What is the durability of the devices? If I have to replace my heating every 20 years, I don’t gain much if the break-even is only after 15 years
- Domestic water consumption is stated as normal, what does that mean? No bathing allowed and only 5 minutes shower per person?

As mentioned, an independent heating load calculation still has to be carried out and then I will get separate offers from building services companies.

Well then, let him guarantee that.
He will pay the difference for the first 10 years (of course at the interest rate of the air-to-water heat pump) -

Questions about the calculation:
Why is the base price 205 per year for gas and 100 € maintenance? Is it really that expensive?
What are the operating costs of the system that occur once?
What exactly are maintenance costs?

He assumes 11.1 KkWh and an annual performance factor of 3.9 - not so unrealistic.

There is a €25 maintenance fee for gas, €10 for air-water heat pumps, and €75 chimney sweep fee for gas and €0 for air-water heat pumps.
In total, therefore, €100 for gas and €10 for air-water heat pumps.
I do not know what maintenance for gas is.

How do you arrive at an annual performance factor of 3.9? The energy per m²? Then it would be over 5 for gas, I think you are confusing something. According to Stiebel Eltron, the LWZ has an annual performance factor of 3.4 at a standard outside temperature of 2 degrees and a flow temperature of 35°.

A architect told us during the initial meeting that although it would still be possible to do solar thermal + gas, afterwards (if one did not take a KFW loan), no one would check it. Many of his clients would simply continue to install a gas boiler and forgo anything further. Of course, he has to point out the regulations accordingly, but whether the person still has solar and a controlled residential ventilation system, no one would check anymore.

Is that really credible?

@ Kaspatoo: Thank you for your calculation, very interesting!