ᐅ KfW 70 House: Gas Consumption Too High, What Solar Panel Output Is Needed?

aytex schrieb:
Hello everyone,
we moved into our new build one year ago and can now assess our energy costs for the first time. We have a vague feeling that they are clearly too high. We have discussed this with our builder and are investigating possible causes. However, I would also like to get your opinions. Here are some key details:

• KFW70
• 180m² (1937 sq ft) living area
• ETICS (external thermal insulation composite system)
• Triple glazing
• Underfloor heating
• Solar thermal for domestic hot water AND heating support (approx. 12m² (130 sq ft) on the roof) from COSMO
• 750-liter (198-gallon) tank
• 3-person household (our youngest is 4 years old)

Our first annual statement shows a consumption of 28,500 kWh. Our research online suggests that this is far too high for a KFW70 house. On average, one would expect about 60 kWh/m² per year. Even if you double this value, our consumption would still be 8,000 kWh higher. We also cannot understand to what extent our solar system has contributed to the overall energy balance, given the high consumption figures.

I would appreciate any feedback, perhaps we can exchange comparison values to better understand where our consumption stands on average.

Hi!

I estimate your real consumption for 180 m² (1937 sq ft) and the given house data to be around 7,000–7,500 kWh.

Screed drying alone can account for 6,000–7,000 kWh.

So your actual consumption within these parameters cannot be easily explained. Not even considering the comparatively large hot water storage tank, which naturally consumes more energy (I roughly estimate that the energy required to maintain the tank’s temperature is about 4 kWh/day, so about 1,500 kWh/year without usage, minus solar gains, which do not require gas).

We also have a KFW70 house, about 140 m² (1507 sq ft), with solar only for domestic hot water (300-liter (79-gallon) tank). Our consumption is about 6,400 kWh/year (and we keep it well heated, around 22°C (72°F) indoors). Solar support is minimal—last year very little, the year before somewhat more (around 400–500 kWh). In short, as mentioned here before, solar support can more or less be neglected—just a nice extra.

Therefore, it is very likely that there is some kind of fault in your system. Somewhere around 8,000–10,000 kWh of energy are being wasted. This could be due to burner settings or other essential issues. Your heating specialist definitely needs to take a closer look.

Best regards,
Thorsten

PS: Dumb question of the day: Was the gas meter reading at 0.0 right after installation?

Someone definitely needs to take another look at this.
Often, it’s small, simple mistakes that go unnoticed but end up costing a lot of money.

In our case, the wrong temperature sensor was installed in the solar system (it was an NTC, but it should have been a PTC or vice versa). I noticed this relatively quickly because the pump was running constantly, even at night. This caused the warm domestic hot water to cool down instead (the solar system was not contributing to heating).
With a lighter flame on the sensor and watching the temperature display on the controller, the issue became clear quickly.
If you don’t pay attention and this continues for months or years, especially if a heating storage tank is also connected...

Hello everyone,
I wanted to briefly revisit the topic and share that after four years, we have settled at around 16,000 (sq ft), which I think is reasonable for the living area and now four people.