Most clients hold their energy performance certificate for the first time at handover and treat it as the last stamp before moving in. The number printed on it, however, was decided far earlier: in the shape of the house, in the build-ups of the walls, in the type of glazing and above all in what heats the house and how it makes hot water. The question I hear most often is about class A0 and whether the house will reach it. The answer almost never depends on insulation thickness alone. This article explains what the certificate actually assesses, which number the class is awarded on, why the same fabric lands in a different class with a heat pump than with direct electric heating, how the design-stage assessment differs from the certificate issued after completion, and why the figure on the certificate is not a forecast of your bill.
What the certificate is and when Slovak law requires one
An energy performance certificate documents the energy performance of one specific building, prepared under a single calculation methodology so that two different buildings can be compared with the same yardstick. It is issued for the building as a whole, places it on a scale from A0 to G, gives partial ratings for each place of consumption and lists recommendations for improvement. It is valid for at most ten years and loses validity sooner if the building undergoes a change that affects its energy performance. The framework is Act No. 555/2005 Coll. on the energy performance of buildings; the calculation method is set by its implementing decree and the related STN 73 0540 series of standards.
The obligation to hold a certificate arises in four situations:
- at final approval (kolaudácia) of a new building,
- after a major renovation of an existing building,
- on the sale of a building or part of one,
- on letting to a new tenant.
The owner hands the certificate to the buyer or tenant, and where a certificate exists its indicator belongs in the listing advertisement too. Exemptions are drawn narrowly: they include buildings under 50 m² of floor area, temporary structures, listed cultural monuments and churches. A family house you are building for yourself is not among the exemptions.
Who issues it and what they need from the designer
The certificate is not issued by the architect or by the building authority but by a qualified certifier holding the relevant authorisation and listed in a publicly accessible register. Their work is largely a transcription of your project into a calculation model, so the quality of the inputs directly determines the quality of the result. What they genuinely need from the design:
- build-ups of every envelope element with thicknesses and thermal conductivities, that is, the basis for calculating the U-value,
- envelope areas, heated volume and the energy reference floor area, not the area from the estate agent's listing,
- technical data sheets for windows and doors: whole-unit Uw, solar transmittance, dimensions and orientation,
- the treatment of thermal bridges, ideally calculated for the actual details,
- the heat source including type, output, seasonal efficiency or SCOP and the flow temperature of the system,
- hot water preparation: cylinder, circulation, pipe insulation,
- ventilation and the efficiency of heat recovery,
- on-site renewables, above all photovoltaic output, tilt and orientation.
If a figure is missing, the calculation does not stop. A conservative default is substituted and the house comes out worse than it should. It is the cheapest possible way to lose a class for nothing.
Which number the class is awarded on
Three different numbers get mixed up in conversations about a house's energy performance. They share a unit and sound alike, they are not interchangeable, and only one of them decides the overall class.
| Indicator in kWh/(m²·year) | What it expresses | Depends on the heat source | Sets the overall class |
|---|---|---|---|
| Specific heat demand for space heating | the heat the fabric needs once solar and internal gains are subtracted | no | no |
| Delivered energy | the energy crossing the building boundary and appearing on the bill | yes | no |
| Primary energy, global indicator | delivered energy multiplied by the factor for each energy carrier | yes | yes |
The overall class is awarded on the global primary energy indicator, that is, on the specific primary energy need referred to the energy reference floor area. Specific heat demand is an important input and tells you about the quality of the envelope, but on its own it does not set the class. This single distinction explains most of the misunderstandings that grow up around certificates.
Why the class rewards the heat source as much as the fabric
Primary energy is obtained by multiplying delivered energy by a factor that accounts for losses in extraction, conversion and distribution of the given carrier. Grid electricity carries a factor of roughly 2.2 to 2.7, natural gas about 1.0 to 1.3, biomass 0.1 to 0.2, and self-generated photovoltaic electricity consumed on site is close to zero. The methodology deliberately rewards renewable and efficient sources.
The consequence is decisive. A heat pump consumes electricity with a high factor, but for every kilowatt hour of electricity it delivers three or more kilowatt hours of heat, so delivered energy drops several times over. Direct electric heating carries the same factor and no such multiplier. The identical fabric therefore lands in different classes depending on what heats it.
| Heat and hot water source | Seasonal efficiency or SCOP | Delivered energy | Factor | Primary energy | Position on the scale |
|---|---|---|---|---|---|
| Direct electric heating | 1.0 | 55 | 2.2 | 121 | worst of the four options |
| Condensing gas boiler | 0.95 | 58 | 1.1 | 64 | better |
| Air-to-water heat pump | 3.5 | 16 | 2.2 | 35 | best |
| Heat pump plus photovoltaics, 30 % self-consumption | 3.5 | 11 from the grid | 2.2 | 25 | best |
An illustrative calculation on identical fabric: 40 kWh/(m²·year) of space heating demand and 15 kWh/(m²·year) for hot water, factors taken at the bottom of their ranges, auxiliary energy and lighting excluded. This is not a certificate, it is a demonstration of the mechanics. The last column therefore shows only the ranking of the options, not specific letters: which class a given figure produces depends on the ladder in force at the time of assessment and on the building category. The specific factors and thresholds that apply to your house will be entered by the qualified certifier under the methodology in force.
The table carries a warning as well. A class can be improved by technology without the house becoming a better-built house. The envelope is the one part that lasts fifty years without replacement, while the heat source is replaced after fifteen. That is why it is worth asking about heat demand too, not only about the letter on the certificate.
What class A0 means
A0 is the best class on the scale and in Slovak conditions it is also a regulatory threshold. Since 1 January 2021 every new building must meet the parameters of a nearly zero-energy building, which for family houses means precisely class A0. Between 2016 and 2020 class A1 was sufficient. The energy class is therefore not a marketing badge but a condition of both permission and final approval.
The threshold is set for the global primary energy indicator, and it is not one single figure for every building: it differs by category, because a family house, an apartment block, an office and a school have different use profiles. The numeric values circulating in articles and in marketing material contradict one another, including among sources that present themselves as authoritative, which is why none appears here. The only binding values are those in the implementing decree and in the standards in force on the date the certificate is issued. If you need to know where your threshold actually lies, ask your certifier which ladder they applied to your house.
One more distinction is worth carrying in your head: A0 is not a passive house. The passive standard caps space heating demand and airtightness, that is, what the house needs before any decision about the heat source is made. The difference is set out in detail in Passive House from A to Z.
Design assessment versus the certificate after completion
Two documents arise during preparation and construction, and clients routinely confuse them. They differ in when they are produced and in what they actually assess.
| Feature | Design energy assessment | Energy performance certificate |
|---|---|---|
| When it is produced | before permission, as part of the design documentation | after completion, before final approval |
| What it rests on | the proposed solution | the as-built condition |
| What it is for | demonstrating that the design meets the requirements | a document handed to the owner and on to the buyer |
| What voids it | a change to the design during preparation | a change to the building that affects performance |
The trouble starts when the house as built differs from the design, which happens regularly. Windows swapped for a cheaper type, thinner insulation at the plinth, photovoltaics dropped, a different heat source than the one specified. The certificate has to be prepared on the actual condition, not on the original drawings. If the result slips below the required level, the house fails the requirement for new buildings and it is resolved either by adding measures or by delaying final approval. The other option, a certificate written against a paper design, looks simpler and becomes a problem later, at sale or in the first winter. Three rules that prevent it:
- have every substitution of a material or a device assessed by the designer before it is installed, not after,
- collect technical data sheets continuously during construction, not in the week before final approval,
- if a substitution worsens the result, look for compensation elsewhere in the balance while the site is still open.
The certificate is not a forecast of your bill
The calculation rests on standardised assumptions: 20 °C indoors, a standard occupancy profile, standard hot water consumption, standard ventilation and long-term climate averages for the location. Nobody models the fact that your household prefers 23 °C, that two people live in the house instead of four, or that the wood stove runs every weekend. A household holding three degrees more will exceed the calculated heat consumption by tens of percent, and the certificate remains correct, because it answers a different question.
The second point is that primary energy is not money. The factor of 2.2 on electricity is not a tariff; it is a conversion to the fuel burned at the generating end. A house with a heat pump has an excellent primary energy indicator and you still pay the grid price for every kilowatt hour of electricity. The certificate is a tool for comparing buildings with each other and for demonstrating compliance, not a calculator of running costs. If you want an estimate of the bill, you need a calculation using your own inputs. The economics are covered in Is a passive house worth it.
Conclusion
The certificate is the last document in the process, but the decisions that determine it are taken right at the concept stage: compactness of the massing, the area and orientation of the glazing, the envelope build-ups and the chosen heat source. Ask about the class after final approval and you are merely finding out the result. Ask about it during the concept study and you can still change it, at practically no cost.