What is a DEC?

A DEC shows the energy performance of a building based on actual energy consumption as recorded over the last 12 months within the validity period of the DEC (the operational rating).

The operational rating is a numerical indicator of the actual annual carbon dioxide emissions from the building. The various types of energy consumption from occupying a building must be brought together on a common basis so that the performance of one building can be compared with that of another. The UK has decided that the common unit should be CO₂ emissions, since this is a key driver for energy policy.

This rating is shown on a scale from A to G, where A is the lowest CO₂ emissions (best) and G is the highest CO₂ emissions (worst).

DECs for buildings larger than 1,000m² also show the operational ratings for the previous 2 years, where available.

The operational rating is based on the amount of energy consumed during the occupation of the building over a period of 12 months from meter readings and is compared to a hypothetical building with performance equal to one typical of its type (the benchmark).

Typical performance for that type of building would have an operational rating of 100. A building that resulted in zero CO₂ emissions would have an operational rating of zero, and a building that resulted in twice the typical CO₂ emissions would have an operational rating of 200. If the building is a net energy generator, it would still be given an operational rating of zero.

The operational rating must be calculated according to the methodology approved by the Secretary of State. This is done by an accredited energy assessor using a software tool for the calculation which has been approved by the Secretary of State. See the DEC software specification.

The DEC should be displayed in a prominent place that is clearly visible to members of the public. A sample certificate is shown. To enable members of the public to view the document easily, it should be no smaller than A3 in size.

A DEC must be accompanied by an advisory report and the owner of the building must have a valid one available. The advisory report highlights recommendations to improve the energy performance of the building (i.e. its fabric and associated services such as heating, ventilation and lighting).