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_HH
Hello 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
DeepRed schrieb:
We are talking about around 50 kWh per (summer) month. Just to properly put the statement "needs quite a bit for domestic hot water in summer" into context.
(Single-family house, 3 adults, 2 bathrooms) 1500 kWh per year corresponds to only 120 kWh per month.
So I find an additional 50 kWh already worth mentioning.
With 2 adults and 3 children, we use about 100 kWh per month in summer for domestic hot water.
N
nordanney5 Dec 2025 13:33julimos schrieb:
We are 2 adults plus 3 children using around 100 kWh per month for hot water in summer.One adult with part-time care for 3 children uses roughly 25 kWh in summer — my tenant, a senior single, uses about 35 kWh in summer. 100 kWh seems very high to me — probably indicating a very high water consumption (if not, then a poorly functioning heat pump). For me, that corresponds to about 60 to 70 hot water cycles with the heat pump per month.
nordanney schrieb:
An adult with three children part-time in summer uses roughly 25 kWh in summer – my tenant, a senior single, needs about 35 kWh in summer. 100 kWh seems very high to me – probably a very high water consumption as well (if not, then the heat pump is running extremely inefficiently). For me, that corresponds to about 60-70 hot water preparations with the heat pump per month. Okay, I can’t manage 25 kWh either, but I don’t have any KfW standard house (renovated older property). But yes... 100 kWh per summer month for hot water is too much. Either the pump is not running properly or the three children are all teenage girls. That can definitely mean longer time in the bathroom.
Children shower after the pool, partly adults showering in the morning and evening due to heat, sports, and the pool. The dishwasher is also connected to the hot water system and runs up to twice a day when everyone is home during holidays or weekends. The consumption value is reasonable, and the heat pump operates within the expected efficiency range for hot water (around 3 over the course of the year).
There is a range, of course, and we would definitely manage with less.
And if the electricity is almost free thanks to photovoltaics, then you don’t need to be too concerned about it ;-)
To get back to the topic: If you want to know exactly, you need to monitor your consumption for at least one year, recording it hourly.
To estimate, use my calculation above and adjust it if necessary, but amortization over 10 years is already very good.
There is a range, of course, and we would definitely manage with less.
And if the electricity is almost free thanks to photovoltaics, then you don’t need to be too concerned about it ;-)
To get back to the topic: If you want to know exactly, you need to monitor your consumption for at least one year, recording it hourly.
To estimate, use my calculation above and adjust it if necessary, but amortization over 10 years is already very good.
N
nordanney5 Dec 2025 14:55DeepRed schrieb:
Well, I can’t manage 25 kWh, but I don’t have a KfW-standard house or anything like that (renovated old property).The type of house doesn’t really matter in this case. It’s only about heating domestic hot water. The consumption would be the same even in a property with energy efficiency class H, provided that the pipe to the hot water tank is new and insulated.N
nordanney5 Dec 2025 14:56julimos schrieb:
And when electricity is almost free thanks to photovoltaic systems, you don’t have to worry about it that much ;-)Yep – having air conditioning during the day doesn’t hurt either (I find it very comfortable).Similar topics