Glossary
Energy glossary
Quick definitions of the main terms used in building energy efficiency.
- EPC Energy Performance Certificate
- The generic international term for energy performance certificates for buildings, required by the EU's EPBD (Energy Performance of Buildings Directive). In Italy it is called APE, in France DPE, in Germany Energieausweis.
- APE Attestato di Prestazione Energetica (Italy)
- The Italian energy certificate, mandatory for sales and rentals since 2013 (Legislative Decree 192/2005). Scale from A4 (highest efficiency) to G. Must be prepared by a qualified professional registered with the relevant regional registry, following an on-site inspection.
- DPE Diagnostic de Performance Énergétique (France)
- The French energy certificate, mandatory since 2006. Since 2021 (DPE reform) it uses a dual criterion: the final class is the worse of the energy consumption class and the CO₂ emission class. Scale A–G for both criteria.
- Energieausweis (Germany)
- The German energy certificate, regulated by GEG 2020 (Gebäudeenergiegesetz). Scale from A+ (≤30 kWh/m²·year) to H (≥250 kWh/m²·year). Two variants exist: Bedarfsausweis (based on calculated demand) and Verbrauchsausweis (based on measured actual consumption).
- CEE Certificado de Eficiencia Energética (Spain)
- The Spanish energy certificate, regulated by RD 235/2013. Scale A–G, with a separate indication of CO₂ emissions (kg CO₂/m²·year). Mandatory for sales and rentals.
- EP Primary Energy
- Energy measured before any transformation: it includes transport and conversion losses. For buildings a distinction is made between EPgl,tot (global total) and EPgl,nren (non-renewable). Energy classes are based on non-renewable primary energy per m² per year.
- Heating Degree Days (HDD)
- An indicator of the heating climate severity of a location. Calculated as the sum of daily differences between a reference temperature (typically 18°C) and the mean outdoor temperature on days when it falls below that threshold. In Italy they range from ~500 HDD (Sicilian coast) to ~4000 HDD (high Alpine areas).
- Climate zone
- In Italy, DPR 412/93 divides the territory into 6 zones (A–F) based on heating degree days. Zone A (≤600 HDD, e.g. Lampedusa) is the mildest; zone F (>3000 HDD, e.g. Bolzano) the most severe. The climate zone determines the length and limits of the heating season and the maximum transmittance thresholds for renovations.
- Thermal transmittance U (U-value)
- Measures how quickly heat flows through a building element (wall, roof, window) per unit area and temperature difference: W/(m²·K). The lower the U-value, the better the thermal insulation. A 1970s brick wall typically has U ≈ 1.2–1.5 W/(m²·K); a modern insulated wall can reach U ≈ 0.2–0.3 W/(m²·K).
- DHW Domestic Hot Water
- Hot water used for sanitary purposes (shower, sink, etc.), distinct from space-heating hot water. In energy calculations DHW demand depends on the number of occupants and the type of generator. In Italian energy certificates (APE) it is included in the global EP index.
- Heat pump
- A system that transfers heat from the outside (air, ground or groundwater) to the inside of a building. It has a COP (Coefficient of Performance) typically between 2.5 and 4.5: for each kWh of electricity consumed it produces 2.5–4.5 kWh of heat. Because the primary energy factor for electricity is much lower than for gas, heat pumps significantly improve the energy class compared to traditional boilers.
- Photovoltaic (PV)
- A system that converts solar radiation into electricity. In building energy calculations, electricity produced on-site is subtracted from the gross demand (self-consumption), reducing the EPgl,nren index and improving the class. System size is expressed in kWp (kilowatt-peak). A 100 m² apartment in a sunny location with 3 kWp can gain 1–2 energy classes.