ᐅ Vestaxx window heating – experiences?

Interrupt schrieb:

Sorry, I’m a bit late to the party and am trying to understand the formula. I don’t (yet) have a photovoltaic system on my roof and I’m not very experienced with economic calculations.

@RotorMotor, why are you adding 600 kWh of feed-in tariff that you previously accounted for as self-consumption in the electricity demand? A kilowatt-hour produced by the photovoltaic system can’t be both self-consumed and sold at the same time, right!? Besides that: shouldn’t the feed-in tariff be subtracted instead of added (since it’s income, not an 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 use it to generate heat via the heating system.
Since this concerns heating systems, the lost feed-in tariff due to using the electricity for heating must be recorded as a cost and therefore added to the expenses.
After all, I could also heat with gas, wood, etc., for which I would receive the feed-in tariff.

Perhaps it helps to think of (and often it’s actually the case) the photovoltaic system as a "business."
You invest in photovoltaics and have a system in this "business" that can sell electricity for 8.2 ct/kWh.
However, you can also consume this electricity yourself, which makes sense, because otherwise you would have to buy power from another provider at 35 ct/kWh.
But this means the "business" incurs lost revenue (a loss) equal to 8.2 ct/kWh.

Vestaxx GmbH schrieb:

The calculation is completely wrong simply because it excludes the total yield of the photovoltaic system from Vx + Photovoltaic and takes only the yield that can be used directly for heating in winter. All other electricity produced by the photovoltaic system is simply ignored.

Incorrect, the yields of a photovoltaic system belong in the economic analysis of the photovoltaic system itself, not in that of the heating system.
If you want to continue discussing the profitability of photovoltaic systems, please open a separate thread for that. This thread is about electric panel heaters.
At most, "synergy effects" belong in the heating system’s calculation, and I have fully included these.

face26 schrieb:

In Baden-Württemberg, the argument about photovoltaics is already obsolete since photovoltaic systems are mandatory for new builds.

Even mandatory photovoltaics come with costs. Therefore, it is entirely reasonable to ask where money can be saved during construction to avoid increasing the loan amount further. However, if I move beyond just comparing heating systems, I need to be honest and consider all options related to the house: energy standards, features, carport versus garage, own labor contributions, and so on. With a fixed budget, this creates a variety of possible houses that all lie on a Pareto frontier—meaning you can only improve one aspect by worsening another.

In this context, only heating or heating plus photovoltaics are being discussed. If I plan a house meeting EH55 standard, it currently makes more sense to meet the Building Energy Act (Gebäudeenergiegesetz) requirements and use the money saved to install a photovoltaic system in the range of 6–8 kWp, which is better sized for self-consumption. Feed-in tariffs, with prices close to 2000€ net per kWp, are no longer very profitable. In this approach, although the heat demand might be about one-third higher, the higher efficiency reduces heating costs by roughly one-third—while staying within the same budget.

Another question is whether underfloor heating is truly on the Pareto frontier, even when choosing direct heating. A standard infrared heating system is likely to be considerably cheaper, with flexible placement, higher output over small heating surfaces (for example, bathrooms), and many suppliers. This means it can be replaced after decades even if the original manufacturer no longer exists (see Interpane).

RotorMotor schrieb:

Exactly right. I can either sell the electricity from the photovoltaic system for 8.2¢/kWh or convert it into heat using the heating system.
Since this concerns heating, the lost feed-in tariff must be accounted for as a cost and therefore added to the expenses.

Thanks. That’s not something you’d think of immediately. Now I remember why I studied computer science and not business administration back then. 😀

WilderSueden schrieb:

Mandatory photovoltaics also come with costs. Therefore, it is completely reasonable to ask where you can save money during construction to avoid increasing the loan amount further. However, if I look beyond just comparing heating systems, I have to be honest and consider all options for the house: energy standards, features, carport instead of garage, own labor, and so on. With a fixed budget, this results in a variety of possible houses that all lie on a Pareto front, meaning you can only improve one aspect by compromising another.

That is exactly what I meant. This comparison is irrelevant for Baden-Württemberg when looking at windows plus heating system with photovoltaics versus heat pump plus underfloor heating without photovoltaics. That combination is currently not available in the newly planned single-family house. All subsequent steps including where to save budget are as you already described.

RotorMotor schrieb:

Dear Christian, what exactly don’t you understand about the calculations?
Your entire summary is quite far from the reality of this thread.
We have always based our arguments on numbers and calculations,
not on feelings or general rejections of certain technologies.
We have also never used any sleight of hand tricks like moving money from one pocket to another.

Nor have we ever said that you cannot or must not deviate.
We only strongly disagreed when it was claimed that an alternative approach would be fundamentally more sensible.

Sure, I realize I spend far too much time here, I just noticed. 😉

But that’s exactly why a full cost accounting is so important.
If someone already has a tight budget, they should be especially careful not to bring the wrong technology into their home.
As I have explained repeatedly, it’s not, as V. suggested, only about the initial investment cost.
If instead of paying more to the bank, you end up paying much more to the energy provider, that helps no one.
Neither the homeowner nor the environment!
What really matters is the total monthly expense!

There is a small error in the reasoning here.
A high feed-in tariff is actually a disadvantage in the economic assessment of inefficient heating systems.
Because this means I lose more money by using the electricity myself.
Here again, the heat pump allows me to consume less and thus feed more of the well-paid electricity into the grid.

Feel free to start a separate thread on this topic where we can discuss it further.

First of all, thanks for addressing me so politely now.
Since I currently don’t have a photovoltaic system, I rely on the data available online or in this thread.
I realize that this may not always reflect everyone’s actual situation, but at least it applies to those who have posted here.
And the table in Post 100 (wilder Süden) shows that a 12 kWp system still delivers about 700 kWh per month in the four coldest months, while the total annual generation is around 13,000 kWh. In Post 169 (Ibnan), a 6.5 kWp system produces about 8,500 kWh per year.
If I extrapolate your, in my opinion, realistic values for household electricity consumption at 10 kWh per day and hot water at about 2 kWh per day, I come to an annual electricity demand of 4,400 kWh that I do not need to purchase.
And the file from wilder Süden then says that I can feed the remaining 8,600 kWh back into the grid. At 8 cents per kWh, that’s about 640€ per year.
If this system still produces 700 kWh per month in winter, and I can only use 150 kWh per month for heating during the winter months, I would have to buy 5,400 kWh. At 35 cents per kWh, that’s about 1,890€ (and possibly minus any feed-in tariff).
Someone without a photovoltaic system has to purchase the full 4,400 kWh for household electricity plus 1,500 kWh for heating — 5,900 kWh total — at 35 cents per kWh = 2,065€.
From this point on, it doesn’t matter how expensive electricity becomes.
And all this assumes both systems would cost the same.
A 12 kWp photovoltaic system would likely cost around 22,000€ and an electric heating system about 10,000€?
And a heat pump with underfloor heating would probably be roughly the same price?
That’s basically what I meant.
This makes electric heating not economic, and it can never be more cost-effective than a heat pump.

And yes, the consideration of an electric instantaneous water heater deserves its own thread.

Best regards, Christian

Usually, I work with property buyers who start the conversation with something like: "I have a budget of (for example) 400k and I absolutely do not want to exceed it." When I see that the house only costs 340k, I recommend a heating system with a heat pump combined with photovoltaic panels and an energy storage unit, as well as a small, separate domestic hot water heat pump. This is always dependent on the thermal insulation calculation.

Unfortunately, prices and interest rates have risen, and some people then have to abandon their project. With these clients, I look for alternatives to make the project feasible at all.

Photovoltaics isn’t always possible; sometimes the location of the house is unsuitable or the desired architectural design is unfavorable, leaving only a heat pump without photovoltaic panels as an option. That is just how it is sometimes. But it isn’t always the case.