Operational Energy Use: The Difference Between EPCs and DECs

They both report the energy efficiency in a similar way to the energy efficiency rating of appliances: A building is given a rating between A and G, with A being the most energy efficient. While both certificates indicate the energy efficiency of a building, they differ in several ways, most significantly in how the energy rating is assessed. A certified professional is required to carry out an assessment to produce either an EPC or a DEC.

EPCs are required for all domestic and non-domestic buildings that have been newly constructed, have undergone significant renovation, or are being sold or leased. The certificates are created through modelling, predicting the energy use rather than using measured energy use. To ensure consistency, the modelling must follow the standards outlined within the UK National Calculation Methodology (NCM) and be undertaken using certified software.

There are different software types used for domestic and non-domestic buildings. When assessing domestic buildings, Standard Assessment Procedure (SAP) software is typically used; this is a simple piece of software, with the inputs fed into a spreadsheet that calculates the energy efficiency of the dwelling. For non-domestic buildings there are two different options available for calculating a building’s energy use; Simplified Building Energy Modelling (SBEM) or dynamic simulation modelling (DSM). The SBEM method is similar to SAP for domestic buildings in that it calculates a building’s energy use using a spreadsheet type calculation with averaged monthly weather data, using inputs inserted through the interface (most commonly iSBEM). Due to its simplicity, SBEM is only suitable for simple buildings; for more complex buildings DSM must be used. DSM is much more accurate than SBEM as it models energy use in much smaller time steps so is much more effective at modelling complex systems. While being required for more complex buildings, DSM can also be used for simpler buildings and is often preferred over SBEM due to its improved accuracy.

Irrespective of the method used to calculate the energy use for an EPC, the calculations use standard use and occupancy profiles. This allows EPCs for the same building type to be compared, however EPCs do not represent actual operative energy use. The energy use rating given by an EPC does not include all energy uses in a building; it is limited to what are known as the regulated loads. These loads are based on the energy required for the fixed building services; heating, cooling, hot water, fans and pumps (auxiliary energy) and lighting. In addition, any energy produced through renewable energy systems are included in the calculation.

An EPC is valid for 10 years so long as no alterations are made to the building.

DECs are required for a much smaller selection of buildings; they are only required for non-domestic buildings with a floor area greater than 250m² that are occupied by a public authority or provide a public service.

While EPCs are based on a modelled energy use, DECs are based on the actual annual energy use of a building for the previous year. This is normally obtained from energy bills and/or metering, which means it includes all energy used by the building, both regulated and unregulated loads. Unregulated loads include energy for cooking, industrial processes and electrical equipment (also known as small power); these loads are not regulated by any standards and are influenced heavily by the occupants, which is why they are not included in EPC modelling; they are much harder to predict.

Invariably, the energy use reported on a DEC is higher than that reported on an EPC for the same building. In part this is due to the unregulated loads being included, but quite often the regulated loads are also higher than that predicted when producing an EPC. This is often due to EPC calculations being based on standard use profiles, which very rarely reflect reality, but can also be due to failure to construct and commission a building to the level of performance targeted by the design.

As DECs are based on the actual annual energy use, they are only valid for one year.

• EPCs are required for all domestic and non-domestic buildings that have been newly constructed, have undergone significant renovation, or are being sold or leased, while DECs are required for a much smaller selection of buildings; they are only required for non-domestic buildings with a floor area greater than 250m² that are occupied by a public authority or provide a public service.
• While EPCs are based on a modelled energy use, DECs are based on the actual annual energy use of a building for the previous year.
• An EPC is valid for 10 years so long as no alterations are made to the building, whereas DECs are only valid for a year.