An Energy Performance Certificate (EPC) is a theoretical measure of how efficient your building could be if it were to be operated in a standardised way. This is very unlikely to match how you will be operating your building in reality. An EPC is therefore not representative of how your building will perform in operation and an EPC’s sole purpose is to allow comparisons between buildings to be made. i.e. A building with an ‘A-rated’ EPC would be expected to be more efficient than a similar building with a ‘B-rated’ EPC.
A Display Energy Certificate (DEC) is a measure of how much energy your building has consumed over the last 12 months and therefore reflects your building’s performance in operation. While the EPC and DEC documents look similar, they are in no way comparable and you should not expect a building’s DEC rating to match its EPC rating.