Vestaxx window heating - experiences?

Sorry, I’m a bit late to the party and am currently trying to understand the formula. I (still) don’t have photovoltaics on the roof and am also not particularly skilled in the economic calculation.

, why are you adding 600 kWh of feed-in tariff that you previously counted as self-consumption in the electricity demand? A kilowatt-hour produced by photovoltaics can’t simultaneously be self-used and sold, can it!? Besides that: shouldn’t the feed-in tariff be subtracted rather than added (because it’s income, not expense)? I’m a bit confused here.

Exactly right. I can either sell the electricity from the photovoltaic system for 8.2 ct/kWh or convert it to heat using the heating system. Since this is about heating systems, the lost feed-in tariff caused by the heating system has to be recorded as a cost and therefore added to the costs. After all, I could also heat with gas, wood, etc., for which I would receive the feed-in tariff.

Maybe it helps here (and is often the reality) to think of photovoltaics as a "business." You invest in photovoltaics and thus have a system in this "business" that can sell electricity for 8.2 ct/kWh. However, I can also consume this electricity myself, which of course makes sense because otherwise I would have to buy it from another provider for 35 ct/kWh. However, this causes lost revenue (a loss) for the "business" in the amount of 8.2 ct/kWh.

The calculation is simply completely wrong because he leaves out the total yields of the photovoltaic system in Vx + Photovoltaics and only takes the yield that can be used directly for heating in winter. Everything else that the photovoltaics produce is simply omitted.

Wrong, the income of a photovoltaic system belongs in the economic calculation of the photovoltaic system and not in that of the heating system. If you want to continue discussing the profitability of photovoltaic systems, please create a separate thread for that. This is about window heaters here. At most, "synergy effects" belong in the calculation of the heating system, and I have fully included those.

In BW, the argument with photovoltaics is anyway obsolete, as photovoltaics are mandatory for new buildings.

A mandatory photovoltaic system also costs money. In this respect, it is quite legitimate to ask where money can be saved during construction in order not to further increase the loan. But if I move beyond the pure heating comparison, then I must also be honest and look at all options for the house: energy standard, equipment, carport instead of garage, own work, etc. With a fixed budget, this results in a variety of possible houses that are all on a Pareto front, meaning you can only improve one aspect further by worsening another aspect. But here, only heating or heating+photovoltaics are considered. If I plan an EH55, it currently (at least) makes more sense to reduce it to the Building Energy Act and use the saved money to install a photovoltaic system in the 6-8 kWp range, which is then much better dimensioned for own consumption, since feed-in is probably no longer a good business at prices of almost €2000 net per kWp. Then you have about one third more heat demand but a third of the heating costs due to higher efficiency. With the same budget. There is also the question of whether the window heating really lies on the Pareto front, even if you decide on direct heating. A normal infrared heating system is probably significantly cheaper, can be freely placed, offers higher output on small heating surfaces (e.g., bathroom), and has a variety of suppliers, thus the possibility to replace it in decades even if the original provider no longer exists (see Interpane).

Exactly right. I can either sell the electricity from the photovoltaic system for 8.2 ct/kWh or have it converted into heat by the heating system.
Since this is about heating systems, the lost feed-in tariff must be listed as a cost by the heating system and thus added to the costs.

Thanks. You have to come up with that first. Now I remember again why I studied computer science back then and not business administration. :D

Mandatory photovoltaic systems also cost money. Therefore, it is quite legitimate to ask where money can be saved during construction in order not to further increase the loan. But if I step out of the pure heating comparison, I must also be honest and look at all the options on the house: energy standard, equipment, carport instead of garage, own work, etc. With a fixed budget, there is a multitude of possible houses all lying on a Pareto front, meaning you can only improve one aspect by worsening another.

I didn’t want to express anything else. Obsolete for BW in terms of comparison window heating + photovoltaic vs. heat pump + underfloor heating without photovoltaic. The combo does not currently exist in the newly planned single-family house. All follow-ups including where to save budget then as you already wrote.

Dear Christian, what don’t you understand about the calculations?
Your whole list also misses the reality of this thread by a wide margin.
We have always argued with numbers and calculations.
Not with feelings and certainly not with fundamental rejections of certain technologies.
We have also not used sleight of hand tricks moving money from one pocket to the other.

We have also never said that deviations are not allowed or possible.
We only vehemently disagreed when it was claimed that a different approach would be fundamentally more sensible.


Sure, unfortunately I’m spending way too much time here, I just realized. ;)


But that is exactly why a full cost accounting is so important.
If you are already short on money, you should especially not bring the wrong technology into the house.
It’s simply, as explained multiple times, not exclusively about the investment costs as portrayed by V.
If I end up paying clearly more to the energy supplier instead of the bank, nobody is helped with that.
Neither the homeowner nor the environment!
What matters is ultimately the total monthly burden!


There is a small error in thinking here.
A high feed-in tariff is rather a disadvantage for the economic consideration of inefficient heating systems.
Because that means I lose more money when using it.
Here too, the heat pump allows me to consume less and thus feed in more of the well-paid electricity.


You can gladly open your own thread on this topic to discuss it.

First of all, thanks for addressing me so nicely now.
And since I don’t have a photovoltaic system at the moment, I just use the data that is available on the web or in this thread.
I realize that this does not always correspond to reality for everyone, but at least for those who have posted here.
And the table in post 100 (wilder Süden) shows for a 12 kWp system about 700 kWh per month in the 4 cold months with a total yield of 13,000 kWh, and in post 169 (Ibnan) for a 6.5 kWp system about 8,500 kWh per year.
If I extrapolate your, in my opinion, realistic values for household electricity of 10 kWh/day and hot water of about 2 kWh/day, I come to an annual electricity demand of 4,400 kWh, which I do not have to purchase.
And the file from wilder süden then says that I can feed the remaining 8,600 kWh into the grid. At 8 ct that is about €640 per year.
And if this system still delivers 700 kWh per month in winter, and I can only put 150 kWh per month in the heating during the winter months, I would have to buy 5,400 kWh. That is about €1,890 at 35 ct (and possibly minus the feed-in tariff).
A person without a photovoltaic system always has to buy the 4,400 kWh household electricity plus 1,500 kWh heating electricity = 5,900 kWh × 35 ct = €2,065.
And from here it doesn’t matter how expensive electricity becomes.
And all of this of course only if both systems cost the same.
A 12 kWp system will probably cost about €22,000? And an electric heating system about €10,000?
And a heat pump with underfloor heating probably the same?
And that’s what I meant actually.
With that, electric heating is not more economical, never compared to a heat pump. It can’t be.

And yes, the consideration of an electric instantaneous water heater requires a separate thread.

Best regards, Christian

Usually, I deal with real estate buyers who start a conversation with the following words: "I have a budget of (e.g.) 400k and definitely don’t want to exceed it." When I see that the place only costs 340k, I recommend a heat pump with photovoltaic and energy storage as the heating method, as well as a small, separate BWWP. Always depending on the calculation of the thermal insulation. Unfortunately, prices and interest rates have risen, and some people then have to abandon their project. And with those, I go in search of alternatives so that it can still work out at all. And photovoltaic does not always work; sometimes the place is so poorly situated or the desired architecture is so unfavorable that only a heat pump without photovoltaic remains. That’s just how it is. But that’s not always the case.