Vestaxx window heating - experiences?

Depending on the insulation, the wall has about 19 degrees C and the glass 18 degrees C (of course, this depends on the outside temperatures and the room air temperature).

This makes the argument about the window being the coldest/warmest spot relatively moot. Incidentally, measuring yourself right now is fairly pointless since we are still in the construction phase (see profile picture ;) ) and the results certainly wouldn’t be reliable.

What you don’t consider is that house electricity (about 3-4,000 kWh) must be purchased all year round and the heat pump doesn’t add a damn thing to that. Yes, the heat pump might save about 3,000 kWh per year – but in return, the photovoltaic system generates about five times that amount annually, and a large portion of that is used directly. Do the math and you’ll be amazed! Or are you worried you might calculate yourself into trouble? ;)

If you read my posts more carefully, you will notice that the photovoltaic system is already a given for us, as is the battery, all nicely subsidized by the KfW by over 40%. Photovoltaics only pay off if you don’t pay the current moon prices (or alternatively if electricity becomes even more expensive). But this doesn’t say anything about a particular heating system. As I said, you could also use air heating. Or install infrared heating everywhere yourself. You have to sell your product against this competition, less so against the heat pump.

We don’t sell heating as such but a window heating system in combination with photovoltaics and a WWWP.

You sell the window heating, but you market the photovoltaics. At this point, I would also like to remind you that this thread now ranks quite high on Google.

I just don't understand how anyone comes to say heat pump OR photovoltaics.

You should simply always do photovoltaics and then decide whether you want to heat with an air-water heat pump, brine-water heat pump, direct heating, etc.

Unless you don't have enough money to afford everything.

I can also only imagine the system in very well-insulated buildings or extensions, tiny houses or modular houses (since water-based systems usually don’t work there).

Can't we just do a cost-effectiveness calculation of window heating (€10k) vs. heat pump (€40k) and window heating with photovoltaics (€40k) vs. heat pump with photovoltaics (€70k)? You have €30k available to cover the higher energy costs. Where is the break even point.

Max, Uwe and Svenja Mustermann have inherited a large family property as well as 500,000€ in cash each. Unfortunately, father Mustermann wrote in the will that the payout of the cash assets is linked to a purpose, namely: each must build their own house on the property for exactly this sum, otherwise the money won’t come through. To save money on the architect and structural engineer, they all decided to simply build the same house three times. Exactly the same flat-roofed box with 120 sqm of living space and a volume of 10 x 15 x 3m. They all also choose the same wall and ceiling construction, as well as windows with the same U-value. The energy consultant calculates an average U-value of 0.3 and an envelope surface of 450 sqm. Max is the heat pump specialist and, like the others, has 40k available for heating provision. He decides on an air-to-water heat pump with a COP of 4.2 and pays 9,000€ for underfloor heating and 24,600€ for the heat pump including all ancillary services. In total 40,000€ gross. Uwe has a window surface heating installed for 15k and a photovoltaic system with 12kwp for 25,000€. Svenja thinks both are nonsense and although she installs the same heating technology as Max, she takes out an additional loan of 9,000€ and also installs a photovoltaic system with 4.5kwp. After one year, they all sit in the garden and put the balances on the table. In the 4 cold months (almost always zero degrees outside temperature), like the others, Max needs 450 sqm x 0.3 W/sqm x k x 25k x 10h heating time = 33.7 kWh of heating energy. Since in winter at 0 degrees the heat pump only has a COP of 2, he has to buy 17 kWh of electricity per day. That is 17 kW x 0.3€/kWh x 120 days = 612€ heating energy cost. Uwe needs double the electrical energy, i.e. 34 kWh/day, but his photovoltaic system with 12 kwp has an efficiency of 0.6 during these months with 6 hours of sun and thus produces 43 kWh per day. Since he cannot store energy, he takes 6 h x 3.4 kw/h = 20.4 kWh directly from the PV system and feeds the surplus of 20.6 kW into the grid. For this he receives 8 ct/kWh = 1.64€/day. Unfortunately, he has to pay dearly for the missing 13.4 kWh, namely at 30 ct/kWh, = 4.02€/day minus 1.64€ feed-in tariff = 2.38€/day. That makes 120 days x 2.38€ = 285.6€ for these 4 months in heating energy costs for him. Svenja has her own calculation: she also needs 17kWh/day for the heat pump like Max, but produces only 16.2 kWh/day with the small photovoltaic system (= 2.7 kw/h). This allows her to operate the heat pump for 6 sun hours and feed 1 kw/h surplus into the grid during these 6h and receives 48 ct compensation for this. Unfortunately, she has to buy the missing 4 h x 1.7 kw = 6.8 kw at 30 ct/kWh. That makes 2.04€/day. She paid only 187.20€ for heating energy in these 4 months, but plus 9,000€ x 3.5% interest = 315€/year. During the rest of the year, the balance shifts more and more towards "larger photovoltaic system." I will spare myself the consideration with hot water. Regards Christian

I don't even know where to start with this story... with the COP=2 in the new building over the entire winter? With the assumption that every day has 0 degrees for 4 months? ... that the photovoltaic system produces the average every day? ... that sun and high heating demand tend not to correlate well? ... that Max is the only one who actually has hot water because the others were too stingy for the appropriate device? ... that Max also benefits from COP>>1 when it comes to hot water? ... that 43 kWh/day in winter is more than optimistic even for southern Germany?


And what's the whole fuss about the envelope area when you can simply set a demand per area, optionally between 25 kWh/sqm (EH40) and 45 kWh/sqm (Building Energy Act)? That then results in a heating demand between 3000 kWh and 5400 kWh.

I don't know where to stop. Then calculate with a Delta k of 15 degrees and an average COP of 3. That fits better then, doesn't it?

And only with a yield of an average of 24kwh/day.