KFW, heating demand, heat pumps, consumption

Hello,

we are slowly starting to deal more intensively with the topic of house construction.

In doing so, we repeatedly come across different statements regarding whether KFW is worthwhile or not.

The current subsidies are really substantial. So 18,000,- for KFW55 and 24,000,- for KFW40.

In our case, we definitely want a central controlled residential ventilation system as well as a heat pump. Gas is out of the question, so we consider the heat pump to be the most sensible option.
A photovoltaic system will definitely be installed on the roof (unless it becomes totally uneconomical due to political framework conditions).

Now I’m simply trying to find out how much heating energy I can save with a KFW house. (I will only really know the additional costs when we talk to the construction companies).

We are assuming a house with 145 sqm of living space, without a basement. One full floor, one converted attic with a rather low knee wall of 50 cm (annoying development plan).

Now I’ll start calculating....

KfW-Efficiency House 70	≤ 45 kWh/(m²·a)
KfW-Efficiency House 55	≤ 35 kWh/(m²·a)
KfW-Efficiency House 40 (Plus)	≤ 25 kWh/(m²·a)

This results in the following values for a living area of 145 sqm:

KfW-Efficiency House 70	6525 kWh/a
KfW-Efficiency House 55	5075 kWh/a
KfW-Efficiency House 40 (Plus)	3625 kWh/a

This is the heating energy I need to heat the house.

Now we assume a heat pump with an annual performance factor of 3.5, then I need the following amounts of electricity to heat the respective house:

KfW-Efficiency House 70	1864 kWh/a
KfW-Efficiency House 55	1450 kWh/a
KfW-Efficiency House 40 (Plus)	1036 kWh/a

Now we assume that one kWh of electricity costs 30 cents.
(Yes, photovoltaic power can partially provide cheaper electricity, but most of the electricity for heating will still be needed when the photovoltaic system produces the least, so I just want to calculate "conservatively" now, only with electricity from the grid)

KfW-Efficiency House 70	559 €/a
KfW-Efficiency House 55	435 €/a
KfW-Efficiency House 40 (Plus)	310 €/a

Thus, KFW40 saves around 250€/a compared to the Energy Saving Ordinance,
and KFW55 saves around 125€/a compared to the Energy Saving Ordinance.


Is that correct or is there a mistake somewhere?

Because with this I can actually fairly easily calculate from how many years it would be worthwhile, if I have the surcharge for the respective KFW standard. (Of course minus the previous subsidy).

Is this somewhat applicable or do I have any major conceptual or calculation errors somewhere?

Thanks already!

Best regards
Specki

Would that be correct or is there a mistake somewhere?

Mathematically, that sounds good. Practical benefit, however, tends to zero. Why? Because consumption costs are determined more by you (heating behavior) and the heating technology than by the house. You can easily have higher heating costs in a calculated efficiency house 40+ than in an efficiency house. For example, my last house (built 2014) was by today's standards an efficiency house with 300 sqm floor area and only consumed about €400 in heating costs per year ==> geothermal, controlled residential ventilation, approx. 22 degrees room temperature. Mathematically, much higher heating costs were calculated. Pragmatic approach: What do you want to have in your house? Your statement is controlled residential ventilation + heat pump + photovoltaics. Just have your dream house planned and the KfW standard calculated that you achieve with the technology. Then additionally the extra cost to the next/next but one level. With this information, you can compare for yourself whether the extra cost is worth it considering the offsetting subsidy. You cannot rely much on concrete / calculated consumption.

Estimate the heat demand of your specific house based on a KfW level? There are many generalized parameters involved in the KfW calculation. I wouldn’t use it for a heating load estimation. What good are supposed savings of the standard house to you? Besides, you can’t know the dynamics of electricity costs over the next 10 years; only the direction seems clear, upward... In general, one could also say that with solid construction, the Energy Saving Ordinance standard is economical. Timber frame construction, depending on the standard wall structure, already includes the envelope at KfW 55 or 40 standard without additional costs. So what now? Before you start calculating, more information is needed.

So I really can’t take personal user behavior into account since that’s not even possible; that’s why I wanted to assume a "standardized" house with 145 sqm. And also the requirement of controlled residential ventilation. And of course, you can operate that differently, one person runs it on a low setting, another runs it five times as much as necessary. You can’t capture all that, so I just wanted to rely on standardized facts and set things like heating technology, heating behavior, etc. all equal for now.

Your pragmatic approach is good, but it’s just not thought through completely. How am I supposed to compare in the end if it’s worth it? Say there remain 6,000 euros in additional costs after deducting the subsidy from the Energy Saving Ordinance to KFW55. Then I somehow have to determine how long it (theoretically) takes to recoup the 6,000 euros. And THAT’s where my calculation comes into play. That’s how it was meant to be. And as I said, I know that the values are naturally subject to greater individual fluctuations. But I have to start from something.

Yes, you can also include electricity increase in the calculation. But that too is a bet on the future that you can only estimate roughly.

KF does not pay off through savings. That is ideology. wanting to be part of the solution, not part of the problem.

says the 40+ owner.

on the other hand, we need 2000 kWh of electricity per year for hot water and heating. Neighbor needs 12,000 kWh of gas per year for the same. but my house is 1/3 larger.

[…] whether it is worth it. […]

wrong approach. if it were worth it, there would only be kfw40+ houses. it’s simply not worth it. just like it’s not worth driving a Tesla. but you just want to.