Business Energy Glossary
What is a DEC (Display Energy Certificate)?
A Display Energy Certificate, shortened to DEC, shows the actual operational energy use of a public building over the past 12 months, rated A (most efficient) to G (least efficient). Unlike the Energy Performance Certificate (EPC), which reflects modelled energy efficiency based on construction and equipment, the DEC reflects what the building actually consumed in operation. DECs are mandatory for public buildings in England and Wales over 250 m² that are frequently visited by the public, and must be displayed prominently in the building entrance.
DECs apply to a narrower set of buildings than EPCs but provide more useful operational information. Understanding the difference between EPC and DEC matters for property investors, public sector landlords, and energy managers tracking actual building performance.
On this page
What a DEC shows
A Display Energy Certificate shows:
- Operational rating (A to G). Based on actual energy use over the past 12 months, normalised to building floor area and use type.
- Previous year ratings. Comparison to the prior two years showing trend.
- Annual energy use. Total electricity and fossil fuel consumption in kWh, plus CO2 emissions.
- Floor area. The DEC reference floor area used for the rating calculation.
- Advisory report. Recommended actions to improve operational efficiency.
The certificate provides a snapshot of how well the building is being operated, not just how it was designed.
Who needs a DEC
DECs are mandatory for:
- Public buildings over 250 m² total useful floor area.
- Buildings that are frequently visited by the public.
- Buildings occupied by public authorities (central government, local government, schools, hospitals, etc.).
Typical examples include schools, hospitals, museums, libraries, leisure centres, town halls, and government offices. Buildings under 250 m² do not require a DEC. Private commercial buildings do not require a DEC (they need an EPC at transaction triggers).
How DEC ratings are calculated
The DEC calculation:
- Collect annual energy consumption (electricity in kWh, gas in kWh, other fuels) for the past 12 months.
- Calculate CO2 emissions using DESNZ fuel emission factors.
- Normalise emissions to building floor area and use type.
- Compare normalised emissions to benchmark for the building type.
- Assign rating from A (best performance against benchmark) to G (worst).
The benchmark is set by CIBSE for each public building type (school, hospital, etc.). A rating of 100 indicates exactly typical performance for that building type. Lower scores are better performance.
DEC vs EPC
| Feature | DEC | EPC |
|---|---|---|
| Basis | Actual operational energy use | Modelled energy efficiency |
| Trigger | Annual (or every 10 years for small public buildings) | Sold, let, or built |
| Required for | Public buildings over 250 m² | Commercial property transactions and lets |
| Validity | 1 year (or 10 years for small public buildings) | 10 years |
| Rating reflects | How the building is operated | How the building was designed and built |
Both DEC and EPC use an A to G rating scale but measure different things. A building can have a good EPC rating but a poor DEC rating if it is operated inefficiently, or vice versa.
Display requirements
DECs must be displayed prominently in the building entrance, visible to building users and visitors. Failure to display a valid DEC can result in a £500 fine. The advisory report (with recommended actions) must be retained by the building operator but does not need to be displayed.
For larger public buildings (over 1,000 m²) the DEC must be updated annually. For smaller public buildings (250 m² to 1,000 m²) the DEC must be updated every 10 years.
Using DEC data
Practical uses of DEC ratings:
- Year-on-year tracking. Whether building operation is improving or worsening.
- Benchmark comparison. How the building performs against typical buildings of its type.
- Identifying inefficient buildings. Low DEC ratings flag candidates for energy audit and improvement.
- Capital prioritisation. Decisions on retrofit budgets often draw on DEC trends.
- Public accountability. Building users can see how their public buildings are performing on energy.
The DEC advisory report identifies specific actions and is often the starting point for operational improvement programmes.
a DEC (Display Energy Certificate) FAQs
What is a Display Energy Certificate?
A certificate showing the actual operational energy use of a public building over the past 12 months, rated A (most efficient) to G (least efficient). Mandatory for public buildings over 250 m² in England and Wales.
Who needs a DEC?
Public authorities operating buildings over 250 m² with frequent public visits. Examples include schools, hospitals, museums, libraries, leisure centres, town halls, and government offices.
What is the difference between a DEC and an EPC?
A DEC reflects actual operational energy use over the past year. An EPC reflects modelled energy efficiency based on construction and equipment. A building can have a good EPC but poor DEC if operated inefficiently.
How is a DEC rating calculated?
Annual energy consumption is collected, CO2 emissions calculated using DESNZ factors, normalised to floor area and building type, and compared to CIBSE benchmarks. A score of 100 indicates typical performance for that building type; lower is better.
How long is a DEC valid?
1 year for buildings over 1,000 m² (annual renewal required). 10 years for buildings 250 m² to 1,000 m².
Where must a DEC be displayed?
Prominently in the building entrance, visible to building users and visitors. Failure to display a valid DEC can result in a £500 fine.
Does my private commercial property need a DEC?
No. DECs apply only to public buildings. Private commercial property needs an EPC at transaction triggers (sale, let, new build, major refurbishment) but not a DEC.
How do I improve my DEC rating?
Operational improvements: BMS optimisation, lighting controls, HVAC schedules, occupant engagement, and energy data monitoring. The DEC advisory report identifies specific actions for your building.