1. Why divergence is to be expected
Two documents serve two different purposes, and neither can fulfil the other's. One compares buildings, the other describes use.
The calculated certificate uses standardised assumptions so that buildings can be compared with one another, whereas actual consumption depends on the particular use, which is not standard. The assumptions describe no particular household.
According to professional sources, equating the calculated certificate with actual costs should not be an objective in itself: the original purpose of minimum requirements is to build and renovate buildings efficiently, not to estimate future costs. The divergence is a property of the system.
2. Four sources of divergence
All of them are legitimate rather than faults, and knowing them makes the conversation with a client easier. The causes can be explained.
| Source | Example |
|---|---|
| Number of occupants and their behaviour | same building, different household, different consumption |
| Desired indoor temperature | may differ from what the calculation assumes |
| Actual weather | differs from normal-year data |
| Pattern of use | how many rooms are actually heated |
The third source is particularly relevant, because a normal year is not a fixed quantity. According to information published by the Ministry of Climate, heatwaves have become more frequent and average temperatures have risen, and the degree-day dataset has been updated.
3. What the calculated certificate actually evidences
One sentence resolves most of the confusion, and it is worth saying to the client verbatim. A certificate is not a consumption forecast.
The ETA evidences that the building meets the minimum energy performance requirements. It does not evidence what the building will cost to run.
Compliance is a procedural question, a cost forecast an economic one. Anyone using the ETA to forecast costs is using the right document for the wrong task.
4. How to explain it to a client
Three sentences usually suffice, and their order matters. They cover the assumptions, the use and the weather.
The calculated certificate says whether the building meets the requirements. Actual bills also depend on how much and how the building is used. After two years, actual data exist and give a more precise picture.
The third sentence is the most useful, because it does not brush the question aside but gives it a date. A client who knows when they will get a more precise answer does not read the absence of one as evasion.
5. Why this matters more now
One observation concerns the 2025 changes, and it runs counter to intuition. The methodology moved closer to actual use.
The limit values for energy classes have been tightened, as covered by the article on the new scale. According to professional assessments, lowering the limit values may increase the difference between the ideal situation and actual costs.
Explaining the divergence therefore becomes more important rather than less, precisely as the requirements grow stricter. Expectations of accuracy are rising.
Summary and four practical rules
The calculated certificate uses standardised assumptions for the sake of comparability while actual consumption depends on use, so divergence between them is to be expected rather than an error. Four sources explain it: the number of occupants and their behaviour, the desired indoor temperature, actual weather and the pattern of use. The ETA evidences compliance, which is a procedural question, not costs, which are an economic one.
Four rules: state the divergence beforehand, not after the first bill. Do not use the energy class as a cost forecast. Explain that standardised assumptions are the condition of comparability. Direct the cost question to actual data, available after two years.
This article offers professional orientation as at the date of verification. It replaces neither the regulations in force nor the assessment of a competent specialist.