Energy Class (A0–G)
A harmonized European classification system from A0 to G that rates a building's energy efficiency based on annual primary energy consumption. Class A0 represents nearly zero-energy buildings, while G marks the least efficient. This scale became mandatory across the EU from 29 May 2026.
Energy class is a single, standardized letter rating (A0 through G) that communicates how efficiently a building converts energy into comfort. It is one of the most important pieces of information a buyer, tenant, or regulator needs about a property. Since 29 May 2026, all 27 EU member states use the same eight-point scale to rate buildings, making comparisons possible across borders — a far cry from the fragmented national schemes that existed before. For residential design in Slovakia, understanding energy class has shifted from optional marketing language to a hard regulatory requirement under the new Building Act 25/2025, which came into force on 1 April 2025.
What do the energy classes A0 through G represent?
The eight classes form a hierarchy from best to worst, each roughly representing a doubling of energy demand. Class A0 is reserved for buildings that beat the zero-energy threshold by at least 20% — they produce more renewable energy on-site than they consume annually. Class A represents true zero-energy buildings using no fossil fuels on site. Classes B through F describe progressively higher energy consumption, and Class G represents the poorest-performing buildings in a given country's stock at the moment the scale launches.
The key insight is that Class G is not a fixed absolute standard — it shifts from country to country. In Slovakia, a Class G building will be different from a Class G building in Germany, because G always means "among your nation's worst performers." This anchors the scale to reality and ensures it remains relevant as the whole building stock improves over time.
| Energy Class | Description | Approximate kWh/m²/year range (Slovakia, 2025) |
|---|---|---|
| A0 | Zero-emissions plus — exceeds zero-energy by ≥20%, net-positive renewable generation | Up to 54 |
| A | Zero-emissions — no fossil fuels on site, highly efficient | 55–108 |
| B | Very high efficiency, meets modern building codes | 109–216 |
| C | Good efficiency, moderate energy demand | 217–324 |
| D | Moderate to high energy demand | 325–432 |
| E | High energy demand | 433–540 |
| F | Very high energy demand | 541–648 |
| G | Worst-performing buildings in national stock | Over 648 |
How is energy class calculated from primary energy?
Energy class is calculated from primary energy — the total energy required to generate the delivered energy the building actually uses. Delivered energy includes electricity for lighting and appliances, heat for space heating and water, and cooling energy. Primary energy factors convert these delivered amounts into raw-source equivalents, accounting for generation and distribution losses. A building with solar panels, for instance, consumes zero delivered electricity from the grid for that fraction of demand, but the calculation recognizes that manufacturing, transporting, and installing the panels required embodied energy.
Once annual primary energy consumption is calculated, it is normalized to building area in kWh per square meter per year (kWh/m²/year). This allows fair comparison between a 100 m² apartment and a 500 m² house — both can achieve the same energy class if their per-unit efficiency is equivalent. The classification thresholds vary by country and are set by each member state, but Slovakia's thresholds are defined in the STN 73 0540 thermal protection standard and the new Building Act legislation.
What is an Energy Performance Certificate and where do I need one?
An Energy Performance Certificate (EPC) is the formal document that states a building's energy class along with detailed analysis of its systems, proposed improvements, and cost-benefit estimates. The EPC is legally required when a building is sold, rented to a new tenant, newly constructed, or undergoes a major renovation affecting over 25 % of the building envelope.
In Slovakia since 2008, every residential property must carry a valid EPC. As of 1 April 2025, new buildings must be certified in the new A0–G system and achieve at least class A (nearly zero-energy) under the Building Act 25/2025. The certificate is prepared by a qualified energy auditor and is valid for 10 years (for classes A–C) or 5 years (for classes D–G). After expiration, a new one must be issued, ensuring the classification reflects any renovations or system changes.
How does energy class relate to a nearly zero-energy building?
A nearly zero-energy building (NZEB) is a regulatory definition: a building with very high energy performance whose remaining energy demand is largely covered by renewable sources. In practice, NZEBs typically fall into energy class A or A0. Since 1 January 2021, all new buildings in Slovakia must be NZEBs by law, which means they must achieve class A0 or A. This represents a step change from earlier requirements — older buildings can still legally exist in classes B–G, but new construction has a hard floor at the top of the scale.
This creates an interesting dynamic: energy class A0 represents the regulatory minimum for new residential buildings in Slovakia, not an aspirational target. Achieving lower classes (B–G) is prohibited for new buildings; aiming higher within the A range or exploring Passive House certification becomes a question of value, comfort, and long-term economic sense rather than regulatory compliance.
| Factor | Energy Class | Nearly Zero-Energy Building (NZEB) | Passive House |
|---|---|---|---|
| Primary metric | Annual primary energy (kWh/m²/year) | Very low primary energy + renewable generation | Heating/cooling demand (≤15 kWh/m²/year) + airtightness (n50 ≤0.6) |
| Regulatory status (Slovakia, 2025) | Mandatory classification system | Mandatory minimum for new buildings (class A0) | Voluntary certification |
| Scope | All energy uses: heating, cooling, ventilation, lighting, appliances | Same as energy class, but with on-site renewable sources | Focused on minimizing heating and cooling needs |
| Typical new residential building | Class A0 (≤54 kWh/m²/year primary) | Class A0 requirement since 2021 | May achieve A0 but is separate certification |
What changed when Europe moved to the A–G scale in 2026?
Before 29 May 2026, EU member states used various national rating systems. Some had 10-point scales, others used letters with multiple subcategories (like A1, A2, B, etc.), and the thresholds differed widely. A Class A in one country could mean something very different from Class A in another. This fragmentation made cross-border comparison impossible and trapped property buyers and investors in national information silos.
The recast Energy Performance of Buildings Directive mandated a single, closed eight-class scale (A0–G) valid across all member states. While each country still sets its own thresholds for each class based on its climate and building stock, the structure is now uniform. Renovations prioritized by EU regulation target the worst performers: by 2027, non-residential and public buildings must improve from Class G to Class F; by 2033, from G to Class E. Slovakia is expected to follow these phased targets as part of EU compliance.
What are common misconceptions about energy class?
Myth: Class G is a fixed absolute standard. Fact: Class G is anchored to each country's current stock. As buildings improve, the threshold for G rises, so the label remains meaningful. An older Class G building in Slovakia 2026 may be Class F by 2035 if the average stock improves, but this does not mean the building itself got better — only the reference point changed.
Myth: You can achieve Class A without renewable energy. Fact: Class A requires zero fossil fuels on site. In practice, nearly all Class A buildings include heat pumps, solar thermal systems, or biomass to meet this threshold. A fully electric building using grid power alone almost never reaches Class A unless the grid is almost entirely renewable.
Myth: Energy class tells you everything about comfort. Fact: Energy class measures efficiency but not indoor air quality, thermal comfort, acoustic performance, or daylight. A Class A building with poor ventilation can still feel uncomfortable. That is why Passive House and other holistic standards exist alongside the energy class system.
Myth: A better energy class always costs much more. Fact: Marginal improvements often cost less than their energy savings justify. Moving from Class C to B may add 5–10 % to construction cost but cuts operating costs by 40–50 %. Crossing into Class A0 does carry a larger premium, but the long-term math still favors it in most markets.
How does energy class work in the Slovak residential context?
In Slovakia, the energy class system is now woven into law through the Building Act 25/2025 and the STN 73 0540 standard. Every new residential building must be certified in the A0–G system and must achieve at least class A0. This is a hard requirement, not guidance. The classification is based on the primary energy indicator calculated during design, cross-checked against actual consumption if the building is certified as a Passive House or NZEB.
For existing buildings and major renovations, the logic shifts. Owners and developers are incentivized (through subsidy programmes such as Obnov Dom and EU Taxonomy alignment) to improve from Class D or E up to Class B or A. There is no legal mandate to upgrade every old building, but energy class increasingly affects property value and insurability. Banks and property valuators now price worse-performing classes as liabilities, while Class A buildings command a premium in the residential market.
The practical upshot: for architects designing new homes in Slovakia, energy class is not a target to beat — it is a fixed floor at A0. The question shifts to whether to optimize further for Passive House status, whole-life carbon reduction, or other performance dimensions. For older homes, energy class becomes a roadmap for cost-effective retrofit priorities, guiding which systems to upgrade first to chase better classes and lower utility bills.
Frequently asked questions
- What is the difference between energy class A0 and A?
- Class A0 is reserved for zero-emission buildings that exceed the zero-energy threshold by at least 20% and generate more renewable energy than they consume. Class A represents zero-emission buildings using no fossil fuels. A0 is the highest tier, occasionally offered in some EU countries.
- Does an energy class rating expire?
- Energy Performance Certificates (which document the energy class) are valid for 10 years for classes A–C and 5 years for classes D–G. After expiration, a new assessment is required if the building is sold, rented, or undergoes major renovation.
- How is energy class calculated from kWh/m²?
- Energy class is determined by the building's total annual primary energy consumption per square meter (kWh/m²/year), which includes heating, cooling, ventilation, hot water, and lighting. Member states set their own thresholds for each class based on their national building stock and climate.
- Is energy class the same as a passive house rating?
- No. Energy class is a regulatory classification system measuring total primary energy. Passive house is a specific performance standard focusing on heating/cooling energy and airtightness, with its own certification. A passive house typically achieves energy class A or higher, but the criteria are different.
- When did Europe switch to the A0–G scale?
- Member states were required to transition to the harmonized A0–G scale by 29 May 2026, under the recast Energy Performance of Buildings Directive (EPBD). Some countries received extensions until 2029. Slovakia implemented the change as part of the new Building Act (25/2025 Z. z.), effective 1 April 2025.
- Does achieving a better energy class reduce building costs?
- Higher energy performance typically requires more investment upfront in insulation, systems, and renewables, raising construction costs. However, lower energy classes mean higher operating costs and reduced property value. Over the building's lifetime, energy-efficient classes (A–B) almost always deliver better economic returns.