An Energy Performance Glossary

This glossary explains common energy performance terms in plain language, in context, rather than as abstract definitions. Terms are grouped by theme: energy quantities, building fabric, heating systems, ratings, and retrofit process, because a term's meaning often depends on its theme. A term such as load, for example, means heating demand in one context and electrical load in another, and context is part of the definition.

Use this glossary as a reading aid, not as a technical manual. Where a term has a precise regulatory definition in a particular country or method, that definition governs for legal or assessment purposes, and this plain explanation should be checked against it before being relied on in a transaction or a specification.

Energy quantities come first. Demand is how much energy a home needs, measured in kilowatt-hours. Power, in kilowatts, is the rate of use at a moment. Efficiency is useful output divided by input, so a boiler's efficiency shows how much fuel becomes useful heat. Primary energy includes upstream losses in supplying fuel, while delivered energy is what a meter records. A U-value shows how readily an element lets heat through, with lower better, while airtightness shows how much air leaks through fabric.

Fabric and systems terms follow. Fabric means the envelope: walls, roof, floor, windows and doors. Thermal bridging means heat bypassing insulation at junctions. A thermal bridge is a cold spot where bridging occurs. Retrofit means improving an existing home, as opposed to new build. A heat loss calculation adds fabric and ventilation losses room by room to size heating. Commissioning means setting up and testing a system after installation, so it works as designed.

Rating and process terms complete the set. An EPC is a standard certificate of modelled performance. An audit is a detailed diagnosis of a particular home. A recommendation is a generated or advised measure, not a specification. A baseline is a dated starting record. A renovation roadmap or passport, where national schemes provide one, is a staged plan for improving a home over years. Each term should be read with its document: a recommendation on an EPC is generated, while a recommendation in an audit report is diagnosed, and the two carry different weight.

Check a term in its document before acting on it. Note the sentence around the term, whether the figure it describes was measured, modelled, estimated or assumed, and what units it uses. A term understood in isolation but misread in context, such as confusing a modelled cost with a bill, or a primary energy figure with delivered energy, can lead to wrong decisions despite a correct dictionary definition.

Check national definitions where they matter. Terms such as EPC, audit, retrofit coordinator, and renovation passport have different formal meanings in different countries and schemes, and a term learned from one system should not be assumed to carry the same formal meaning in another. Official glossaries for the relevant system should be consulted for transaction or compliance purposes.

In the UK, glossary terms are shaped by the Standard Assessment Procedure, EPC formats, and retrofit standards, with terms such as SAP rating, RdSAP, and retrofit coordinator carrying specific UK meanings. A UK reader should check those terms against UK guidance, because they do not translate directly to other systems, even where a similar term exists abroad.

In EU member states, terms are shaped by the EPBD and national transposition, with translations adding variation: a term translated from German, French or Polish may not map neatly onto an English word, and a direct translation can mislead. Readers working across languages should check the national definition in the original language where possible, or through an official translation, rather than relying on a general glossary alone.

One glossary mistake is treating a plain explanation as a regulatory definition. A plain explanation helps reading, but a legal requirement, a grant condition, or a specification should be checked against its formal definition, because plain language necessarily simplifies, and simplification can omit a condition that matters for compliance or funding.

Another mistake is using jargon to impress rather than to clarify. A quote, report or conversation full of unexplained terms may sound expert, but it hides assumptions and prevents the householder from judging advice. A good professional defines terms in context, in plain language, and welcomes questions about them, while a professional who hides behind jargon should be asked to explain.

Keep a personal glossary notebook, adding terms as they are encountered, with plain explanations in your own words, examples from your home, and sources. That notebook becomes a reliable reference across certificates, audits and quotes, and it prevents the same term confusing you twice in different documents.

Ask professionals to define any term you do not understand, in the context of your home, and to state what figure or quantity it describes. That habit, repeated, turns jargon from a barrier into a tool, and it is the single most useful reading skill for energy documents. Official glossaries linked below are a good next stop.