While both assess energy performance, a DEC reflects the actual operational energy use of a building, based on real energy consumption data. In contrast, an EPC evaluates a building's theoretical energy efficiency based on its design and construction features, rather than how it is used.

DECs are required for public buildings with a total useful floor area over 250m² that are frequently visited by the public. Examples include schools, hospitals, libraries, and government offices.

A DEC assessment requires:

• Annual energy consumption data (eg, electricity, gas, and heating oil bills).
• Floor plans or building layouts to determine the total usable floor area.
• Details about building use, occupancy patterns, and operating hours.
• Information on installed systems like heating, ventilation, and lighting.
• On-site assessment access to retrieve any of the above information.

A standard DEC assessment in Colchester typically takes 5-7 working days, depending on the availability of energy data and the complexity of the building. Buildings over 1,000m² may require additional time for the advisory report.

A DEC advisory report provides broad recommendations to improve energy efficiency based on operational data, focusing on practical changes like upgrading systems or optimising energy use. An energy audit, however, is a more detailed and comprehensive analysis, often including cost-benefit assessments for specific energy-saving measures.