ᐅ Photovoltaic System for a KfW 55 Semi-Detached House with Heat Pump: Tips on Storage and Orientation Choices
Created on: 4 Dec 2025 22:10
J
JAMO_HHHello everyone,
We have a semi-detached house (new build, KfW 55 standard) with a gable roof facing south-southeast/north-northwest and an electricity consumption of around 2,500 kWh household electricity (currently about 1,500, but our three children are still small...) plus 3,000 kWh for the heat pump. We don’t have an electric car at the moment, but possibly in the future. If it turns out to be cost-effective, we would like to install a photovoltaic system on the south-southeast side – only 11 modules fit there (possibly +1 on the dormer) because we have a dormer, window, and tree on that side, so roughly 5 kWp. I am a complete layperson on the subject, so we obtained four different quotes.
The first said: It does not pay off.
The second said: Definitely worth it, but he would also cover the north-northwest side with an additional approx. 6 kWp and install a battery plus cascade control for the heat pump – that would pay off. (Quote is about $17,000 for approx. 11 kWp; 25x Trina Vertex S+ + 8.76 kWh Sigen battery + 10 kW Sigen Energy Controller inverter) + additional recommendations: Stiebel Eltron ISG Connect for heat pump control using photovoltaic surplus + Sigen EnergyConnect HomePro.
The third said: It’s worthwhile, he would only cover the south-southeast side + battery (quote about $10,000, including 11x Trina Vertex S+ or AIKO Neostar 2S+ + 5 kWh Ecoflow PowerOcean battery + 8 kW PowerOcean hybrid inverter).
The fourth said: It’s worthwhile, he would only cover the south-southeast side but also the dormer and no battery, because that would not pay off – and definitely use German manufacturers, everything else is nonsense because no one is reachable in case of warranty issues (main consumption is in winter when the photovoltaic yield is very low). (Quote about $13,000, including 12x Solyco R-BG 108nBC.6 specially for partial shading + 10 kW Neeom Staak Eco inverter).
(The reason they all suggest somewhat larger inverters is that we might eventually cover a carport still to be built with modules if it makes sense.)
So, I have four completely different opinions on one thing where, given the same assumptions, one would expect similar conclusions and results... I am totally confused. My research as a layperson and my own payback calculations show that a battery is probably not profitable, so I would leave it out initially and possibly add it after 2 or 3 years when we know the exact yield and self-consumption. I also think that a photovoltaic system – if at all – would only somewhat pay off with the cheapest quote for 11 or 12 modules on the south-southeast side.
Am I completely off track or what do you think? I would especially be interested in which basic configuration you would recommend (with or without battery? covering the north-northwest side or not? using Stiebel Eltron ISG Connect or not? definitely German manufacturers or not?) or if it’s better to avoid it altogether?
Best regards and many thanks in advance for any advice, Jan
We have a semi-detached house (new build, KfW 55 standard) with a gable roof facing south-southeast/north-northwest and an electricity consumption of around 2,500 kWh household electricity (currently about 1,500, but our three children are still small...) plus 3,000 kWh for the heat pump. We don’t have an electric car at the moment, but possibly in the future. If it turns out to be cost-effective, we would like to install a photovoltaic system on the south-southeast side – only 11 modules fit there (possibly +1 on the dormer) because we have a dormer, window, and tree on that side, so roughly 5 kWp. I am a complete layperson on the subject, so we obtained four different quotes.
The first said: It does not pay off.
The second said: Definitely worth it, but he would also cover the north-northwest side with an additional approx. 6 kWp and install a battery plus cascade control for the heat pump – that would pay off. (Quote is about $17,000 for approx. 11 kWp; 25x Trina Vertex S+ + 8.76 kWh Sigen battery + 10 kW Sigen Energy Controller inverter) + additional recommendations: Stiebel Eltron ISG Connect for heat pump control using photovoltaic surplus + Sigen EnergyConnect HomePro.
The third said: It’s worthwhile, he would only cover the south-southeast side + battery (quote about $10,000, including 11x Trina Vertex S+ or AIKO Neostar 2S+ + 5 kWh Ecoflow PowerOcean battery + 8 kW PowerOcean hybrid inverter).
The fourth said: It’s worthwhile, he would only cover the south-southeast side but also the dormer and no battery, because that would not pay off – and definitely use German manufacturers, everything else is nonsense because no one is reachable in case of warranty issues (main consumption is in winter when the photovoltaic yield is very low). (Quote about $13,000, including 12x Solyco R-BG 108nBC.6 specially for partial shading + 10 kW Neeom Staak Eco inverter).
(The reason they all suggest somewhat larger inverters is that we might eventually cover a carport still to be built with modules if it makes sense.)
So, I have four completely different opinions on one thing where, given the same assumptions, one would expect similar conclusions and results... I am totally confused. My research as a layperson and my own payback calculations show that a battery is probably not profitable, so I would leave it out initially and possibly add it after 2 or 3 years when we know the exact yield and self-consumption. I also think that a photovoltaic system – if at all – would only somewhat pay off with the cheapest quote for 11 or 12 modules on the south-southeast side.
Am I completely off track or what do you think? I would especially be interested in which basic configuration you would recommend (with or without battery? covering the north-northwest side or not? using Stiebel Eltron ISG Connect or not? definitely German manufacturers or not?) or if it’s better to avoid it altogether?
Best regards and many thanks in advance for any advice, Jan
Good evening, I’m just an amateur as well! But please share the roof pitch!
For example, we have a hipped roof (on a townhouse) with modules on all four roof sides. Our larger roof surfaces face more east and west, with a pitch of 25 degrees, while the other two sides have a pitch of 32 degrees. We don’t have a storage battery. It was too expensive at the time of purchase. The north side also receives sunlight in summer due to the high position of the sun. So please let us know your roof pitch.
Best regards, Jasmin
For example, we have a hipped roof (on a townhouse) with modules on all four roof sides. Our larger roof surfaces face more east and west, with a pitch of 25 degrees, while the other two sides have a pitch of 32 degrees. We don’t have a storage battery. It was too expensive at the time of purchase. The north side also receives sunlight in summer due to the high position of the sun. So please let us know your roof pitch.
Best regards, Jasmin
Have you done the calculations yourself?
I’m quite sure that having a northwest (NNW) orientation at 42° won’t be worthwhile. You’ll only get yield in the summer, and it will be less than 500 kWh/kWp per year.
So, let’s just consider the 5 kW south-southwest (SSW) system.
It will generate about 5 MWh per year (depending on shading and other factors).
With a battery, you can expect to use around 60% of that energy yourself.
(The heat pump also consumes a fair amount for hot water even in summer.)
That results in 3000 × 0.27 + 2000 × 0.07 = 950 Euros per year.
If you can get the system for 10,000 Euros, it should be a good deal.
I’m quite sure that having a northwest (NNW) orientation at 42° won’t be worthwhile. You’ll only get yield in the summer, and it will be less than 500 kWh/kWp per year.
So, let’s just consider the 5 kW south-southwest (SSW) system.
It will generate about 5 MWh per year (depending on shading and other factors).
With a battery, you can expect to use around 60% of that energy yourself.
(The heat pump also consumes a fair amount for hot water even in summer.)
That results in 3000 × 0.27 + 2000 × 0.07 = 950 Euros per year.
If you can get the system for 10,000 Euros, it should be a good deal.
julimos schrieb:
(Heat pumps also require a considerable amount of energy for domestic hot water in summer.)
We are talking about approximately 50 kWh per (summer) month. Just to put the statement "requires a considerable amount of energy for domestic hot water in summer" into the right context. (Single-family house, 3 adults, 2 bathrooms)
On one hand, I had different roof coverings (OSW and OSWN) calculated and then decided whether it was worth it. I would recommend backing up general statements with actual calculations.
With a roof pitch of around 30° (30 degrees), the N option also proved beneficial for us.
We haven’t ordered any storage systems yet, but legislation regarding potential shutdowns and compensation has changed last year. I’m not sure if I would make the same decision now.
I will run the system for one year and analyze day by day how it would have performed with storage and how many years it would take for the storage system to pay off.
However, I have also optimized our energy consumption somewhat for the photovoltaic system without storage.
Since I work remotely twice a week, I charge my hybrid vehicle on those days, and the washing machine and possibly the dryer also run during the day.
I have also adjusted the heating system slightly. In that regard, a storage system would definitely be more convenient.
Although I still need 2.5 months to complete a full year of data, it currently looks like the storage system, as expected, has not yet paid off.
However, storage prices have dropped significantly again in the past year, and the point where the investment pays off within 10-12 years is now close. Then, for us, the only remaining issue would be finding space for the storage system.
With a roof pitch of around 30° (30 degrees), the N option also proved beneficial for us.
We haven’t ordered any storage systems yet, but legislation regarding potential shutdowns and compensation has changed last year. I’m not sure if I would make the same decision now.
I will run the system for one year and analyze day by day how it would have performed with storage and how many years it would take for the storage system to pay off.
However, I have also optimized our energy consumption somewhat for the photovoltaic system without storage.
Since I work remotely twice a week, I charge my hybrid vehicle on those days, and the washing machine and possibly the dryer also run during the day.
I have also adjusted the heating system slightly. In that regard, a storage system would definitely be more convenient.
Although I still need 2.5 months to complete a full year of data, it currently looks like the storage system, as expected, has not yet paid off.
However, storage prices have dropped significantly again in the past year, and the point where the investment pays off within 10-12 years is now close. Then, for us, the only remaining issue would be finding space for the storage system.
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