ᐅ Photovoltaic System and Air-to-Water Heat Pump – Profitability for a Single-Family Home Built to KfW 55 Energy Efficiency Standard

I guess I’m doing pretty well with my 51% for half of November and the entire December 🙂

There is always someone at home here, so that’s not too surprising.

Deliverer schrieb:

My house has a heating load of about 9 kW at the current temperatures (~-4°C (25°F)).
[...]
December around 30%.
28 kWp

I’ve lost track. A 9 kW heating load at -4°C (25°F) is already quite significant and explains a lot. Are there any more details about the (older) building? Then it’s actually within reason, although I’m really impressed by a self-sufficiency rate of 28% in December!

I still charged my car at midday in October with reduced power for higher self-consumption. But I stopped that afterward and now charge with the full 7 kW. It didn’t make sense. On the days when I produce more energy than the heat pump consumes, I turn on the washing machine or put a pizza in the oven.

KingJulien schrieb:

I’m actually doing quite well with 51% for half of November and all of December 🙂
Someone is always home with us, so that’s not too surprising.

That’s really impressive. Do you heat more during the day on purpose? With or without battery storage?
Usually someone is home with us, too, but at these temperatures, I can hardly reduce the heating load at night—at least not without freezing...

OWLer schrieb:

Are there more details about the (old?) building?

I don’t think so, not so far in the forum.
Built in 1962, extension in 1988. Mostly 30cm (12 inches) Poroton brick without insulation. About 260 sqm (2800 sq ft) of living space plus 140 sqm (1500 sq ft) basement. Roof has been renovated, windows from 2000, some insulation between screed and basement ceiling. In 2020, everything was converted to underfloor heating and a heat pump. Calculated heating load is about 13 kW at -10°C (14°F). That roughly matches after two winters of experience. If you want to know more, I suggest a private message so we don’t derail the whole thread. ;-)

I can also compare it to a 2016 new build, close to KfW 55 standard, 135 sqm (1450 sq ft), with a 22 kWp east/west PV system. Since the heat pump is an on-off type and not really persuaded to mainly heat during the day, that house only achieves about 30% self-sufficiency in November and December. Maybe there’s potential for improvement if I look more into smart grid solutions...

Deliverer schrieb:

That’s really impressive. Do you raise the temperature specifically during the day? With or without a battery?

Of course without a battery 😉
Yes, I have a 1.5°C (3°F) night setback from 10 p.m. to 7 a.m. I counteract the cooling down overnight with our masonry heater when it gets cold.
But it also works easily for about 150 m² (1,615 sq ft) KfW55 standard. There is also a modulating heat pump. In December, I think it was around 330 kWh for heating and hot water, without any control system or ventilation (I believe).

Oh, and 23.25 kWp south-southwest and north-northeast orientation in sunny Bavaria. It has been operational since November 11, and today I surpassed one MWh in production.

tomtom79 schrieb:

Ouch, since I’m currently looking into buying a photovoltaic system myself, the 14% figure feels like a slap in the face. Why so low? Why prefer producing hot water instead of feeding electricity into the grid, even if it’s only 7 or 8 cents?

I don’t know, it’s just what came out of the calculations. You have to keep in mind that 4.55 kWp (kilowatt-peak) is not a lot of photovoltaic capacity.
But when it’s REALLY cold outside, the heat pump uses the most electricity. The milder it is, the more efficient it is, and the colder it gets, the steeper the curve rises for power consumption. That’s simply how the technology of these units works, as far as my limited understanding goes.
And on those very freezing days, you basically get nothing from the photovoltaic system.
We had summer months where the consumption covered by photovoltaics was around 40%. But in the summer months, the system only needs a fraction of the electricity it uses in January. In winter, the photovoltaic share is much lower, even if all the electricity generated by the photovoltaic system is fed directly into the air-to-water heat pump.

In summer, we heat the domestic hot water to a higher temperature because it is usually mixed with cold water, for example when showering or bathing. This way, hot water acts as a kind of energy storage. It’s more economical to heat the water with the solar electricity (which I could only sell for about 9 cents) than to keep the water at 40°C (104°F) and then have to heat more water again in the evening, paying around 25 cents per kWh for that process.

You have to look at it more positively: almost 100% of the photovoltaic yield in winter goes directly to heating 😉

In 2021, without much optimization, we had 29% self-consumption with a 9.2 kWp system.