ᐅ KfW (a German development bank), heating demand, heat pumps, consumption

Hello,

we are now starting to take a closer look at the topic of house construction.

We keep encountering different opinions about whether KFW funding is worthwhile or not.

The current subsidies are indeed quite substantial—18,000 euros for KFW55 and 24,000 euros for KFW40.

In our case, we definitely want a centralized mechanical ventilation system with heat recovery and a heat pump. Gas is not an option for us, so a heat pump seems like the most sensible choice.
A photovoltaic system will definitely be installed on the roof (unless it becomes completely uneconomical due to political conditions).

Now I’m trying to find out how much heating energy I can save with a KFW-standard house. (I will only know the additional costs once we speak with the construction companies).

We are assuming a house with 145 sqm (1560 sq ft) of living space, without a basement. One full storey and an extended attic with a rather low knee wall of 50 cm (20 inches) due to a restrictive zoning plan.

Now I’m starting to do some calculations....

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 heating energy requirements for 145 sqm (1560 sq ft):

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 required to heat the house.

Now, assuming a heat pump with a seasonal performance factor (SPF) of 3.5, I calculate the following electricity consumption to heat each house:

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

Let’s assume electricity costs 30 cents per kWh.
(Yes, with photovoltaic you can produce electricity at a lower price, but most of the heating electricity will be needed when the PV system generates the least, so I want to be conservative here and calculate 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

So, compared to the Energy Saving Ordinance (EnEV), KFW40 saves about 250 €/a, and KFW55 about 125 €/a.

Is this calculation correct, or is there an error somewhere?

With this, I could relatively easily calculate after how many years the higher cost for the respective KFW standard would pay off (of course, considering the subsidies first).

Is this a reasonably useful approach, or do you see any major mistakes in my reasoning or calculations?

Thanks in advance!

Best regards,
Specki

Specki schrieb:

Would this be correct, or is there a mistake somewhere?

Mathematically, that sounds fine. However, the practical benefit tends to be close to zero. Why? Because energy costs are influenced more by your behavior (heating habits) and the heating technology than by the house itself.
You can easily have higher heating costs in a theoretically energy-efficient house standard 40+ than in another energy-efficient house. For example, my last house (built in 2014) would be considered an energy-efficient house by today’s standards with about 300 sqm (3,229 sq ft) of floor area and used only around 400€ in heating costs per year — thanks to geothermal heating, controlled ventilation, and a room temperature around 22 degrees Celsius (72°F). The calculated heating costs were much higher.

A pragmatic approach: What do you want your house to include? Your stated features are controlled ventilation + heat pump + photovoltaic panels. Simply have your desired house designed and calculate the corresponding energy efficiency standard (such as the KfW standard) you achieve with that technology. Then, additionally, calculate the extra cost for the next or the next-to-next higher standard. With this information, you can evaluate whether the additional investment makes sense when factoring in the available subsidies. You cannot rely much on specific or calculated energy consumption figures.

Estimating the heat demand of your specific house based on a KfW efficiency level? The KfW calculation includes a lot of general, standardized parameters. I wouldn’t rely on it for determining the actual heating load. What good are supposed savings based on a standard house model?
Besides, you can’t predict the electricity cost trends over the next 10 years; the only clear direction seems to be upwards...
In general, you could say that with solid construction methods, meeting the Energy Saving Ordinance standard is cost-effective. For timber frame construction, depending on the wall build standard, the building envelope can already meet KfW 55 or 40 standards without additional costs.
So, what now?
Before you start crunching the numbers, you need more detailed information.

Personally, I cannot really factor in individual user behavior, as that is not possible. That’s why I wanted to base my calculations on a “standardized” house with 145 sqm (1560 sq ft) and the specification of controlled mechanical ventilation. Of course, this system can be operated differently—one person may run it on a low setting, while another may push five times as much airflow as necessary. None of this can be accurately captured, so I wanted to focus on standardized facts and initially assume the same heating technology and heating habits.

Your pragmatic approach is good but not fully thought through. How am I supposed to compare in the end whether it’s worth it? Let’s say there are additional costs of 6000 (currency) after deducting subsidies from the energy-saving regulation to KFW55. Then I need to somehow determine how long it will (theoretically) take to recover those 6000. This is exactly where my calculation comes in. That was the idea. And as I said, I know the values are subject to wide individual variation. But I have to start from something.

Yes, increasing electricity costs can also be included in the calculation. But that is also a guess about the future that can only be estimated roughly.

The calculation of a heat pump based on savings alone doesn’t make sense. That’s ideology. It’s about being part of the solution, not part of the problem.

says the owner, aged 40+.

On the other hand, we need 2,000 kWh (7,200,000 BTU) of electricity per year for hot water and heating. The neighbor needs 12,000 kWh (41,000,000 BTU) of gas per year for the same. However, my house is three times larger.

Specki schrieb:

[…] whether it’s worth it. […]

Wrong approach. If it were worth it, there would only be KfW40+ houses. It’s simply not worth it. Just like driving a Tesla isn’t worth it financially, but people want to do it anyway.