Gas with solar thermal or heat pump? And possibly photovoltaics?

Hello everyone,

we (a family of 5) are building a solid house in southern Germany (160 sqm living space + basement). Now we have to decide on the heating/hot water solution. Since the question is more about solar/photovoltaics, I hope this question is in the right area here.

We have decided on underfloor heating with gas. To comply with the Energy Saving Ordinance, an additional renewable component is required, such as a combination with solar thermal energy.

After consulting with the house provider, we now have 3 alternatives. Since I am uncertain, I would be very grateful for some comments on our situation.

Option 1:
Gas + solar thermal energy
Total costs: approx. €10,000
Solar thermal energy payback: savings of approx. €300 in heating costs per year. According to the house provider, the solar system pays off in 15-20 years.
I had the impression that this is a worry-free solution that works quite reliably.

Option 2:
Gas (only for underfloor heating) + additional domestic hot water heat pump Brötje BTW 300 (to comply with the Energy Saving Ordinance instead of solar thermal energy)
The domestic hot water heat pump is located in the basement in the technical room and is supposed to heat the domestic water from the ambient air in the basement.
Total costs: approx. €6,000
I like the low total costs. It is clear that we will then have a higher electricity bill due to the heat pump. I wondered whether the system produces enough domestic hot water for 5 people when several shower one after another.

Option 3:
Gas + domestic hot water heat pump (like option 2)
+ photovoltaic system to run the heat pump with part of the electricity
Total costs: approx. €15,000
Photovoltaic system payback: savings/feed-in tariff approx. €1,000 per year. According to the house provider, the photovoltaic system pays off in 8-10 years.
Obviously, the higher total costs are a negative. But if you really "earn" or save €1,000 per year, that would be okay for me.

Question 1: Domestic hot water heat pump Brötje BTW 300
Does anyone have experience with this heat pump or similar systems? Does it produce enough hot water for 5 people? Are these systems mature?

Question 2: Photovoltaic system
So far I have not really dealt with the topic because I thought it no longer makes sense with the low feed-in tariffs. I do not want a battery. If at all, then only photovoltaics on the roof and I use part of the electricity myself and the rest is fed into the grid.
Would you currently still plan a photovoltaic system?

Question 3: Which option would you prefer?
We would like a system that is as low-maintenance, reliable, uncomplicated, and inexpensive as possible.

I am very, very grateful for any feedback

Best regards

Savings of approx. €300 heating costs per year

That is probably a lie. The total gas consumption typically amounts to €600-700 per year, of which 20-25% is accounted for by domestic hot water heating. You can save about 50% of that through solar thermal. Calculate for yourself.

Option 2: Gas (only for underfloor heating) + separate domestic hot water heat pump Brötje BTW 300 (to comply with the Energy Saving Ordinance instead of solar thermal)

I am surprised that this is even approvable. The renewable share must contribute to the heating of the building. This is not given with a separate domestic hot water heating. I am also puzzled by the low acquisition costs of this solution. Did you understand that correctly?

Savings/feed-in tariff of approx. €1000 per year. According to the house provider, the photovoltaic system amortizes in 8-10 years.

This is hard to assess without knowing the system size. But 8 years is again too optimistic, 10 years is more realistic if purchased cheaply.

Either you take gas with solar thermal and possibly photovoltaics if you have the money left over, or simply an air-to-water heat pump and possibly photovoltaics if you have the money left over.

Question 3: Which option would you prefer?
We would like a system that is as low-maintenance, reliable, uncomplicated, and affordable as possible

Either a heat pump or gas + ventilation system with heat recovery. Apparently, not every energy saving ordinance planner can calculate the latter and it is the exception. Nevertheless, it is possible. Ventilation is a comfort gain, other measures are similarly uneconomical and without benefit for you.

A heat pump is somewhat cheaper in maintenance, more ecological/future-proof and would be my favorite. You have to take a bit more care with the heat pump yourself, as the important planning for it is often neglected in practice. Whether heating with a heat pump is ultimately cheaper depends, besides the planning, on how expensive it is sold to you.

Solar thermal is never economical on its own. As large a photovoltaic system as possible is always sensible, independent of the heating system, and pays off through the feed-in tariff after (10-14 years). Battery storage is uneconomical.

We realized KfW70 with gas boiler and controlled residential ventilation including heat recovery upon completion in 2018. I had conduits for photovoltaics installed; I can still retrofit that if needed. Otherwise, nothing else.

PS: 5 people x 30 liters of hot water (from 10 to 55 °) is about 8 kWh per day. At 6 cents/kWh gas, that costs 50 cents per day. With solar thermal you can save about half of that. So in total rather 100 euros per year... or you take very long showers

As far as I have noticed so far, solar thermal energy is not worthwhile, but photovoltaic always is. Have you ever compared the heating costs of gas with those of a heat pump? Don’t just compare the acquisition costs. Especially with regard to CO2 tax, etc. Just the chimney alone that you can save, plus the regular chimney sweep costs.

I am surprised that this is approvable. The renewable share must contribute to heating the building. This is not the case with separate domestic hot water heating.

But it works. If you calculate with a fine-toothed comb and, for example, have a decentralized ventilation system with heat recovery, you can manage solely through heat recovery without solar or domestic hot water heating and still comply with the Energy Saving Ordinance.