Buildings rarely perform as well as their designers predicted – energy consumption can be as much as double what was expected, so annual energy costs can also be doubled. This difference has become known as the performance gap, explained Andy Lewry.

Operators of commercial and public buildings need clear and realistic guidance on targeting energy running costs for their properties and on the potential savings available. At their disposal are two seemingly irreconcilable indicators of performance available: energy performance certificates (EPCs) and display energy certificates (DECs).

WHAT ARE THE RATINGS?

The EPC is a measure of building quality in that it assesses designed performance and is intended to inform people on first occupancy, i.e. at the point of construction, sale or rent, in order to help purchasers or tenants in selecting the right building. However, the asset rating provides no information about how the building is operated in practice and is calculated using standard assumptions.

The DEC records the actual energy use in a building over the course of a year, and benchmarks it against buildings of a similar type. Therefore, to understand and manage the energy use in a building, both ratings are required as they show different aspects of a building’s total energy performance.

Two offices with the same EPC rating could have very different DEC ratings even if they are working under the same operating schedule – a building with a low rating can be used well by its occupants, a building with a high rating can be used wastefully. In the latter case, measures to change the behaviour of the occupants would be the best option for reducing energy use.

 The scenario where a difference is found between the EPC and DEC – IS THE SO CALLED “PERFORMANCE GAP”. This is where the real operation of the building is different to that predicted by design due to the conditions of use being totally different from those standard assumptions.

RESOLVING THE PERFORMANCE GAP

The poor performance of buildings in use compared with their design predictions has been much discussed and various approaches to resolving the problem have been suggested, including:

·           Whole building energy benchmarking

·           More complex modelling

·           Practical analysis of building energy

·           EPCs/DECs – use existing data and tools – can be used as a first cut to target issues or prioritise a portfolio of buildings. We will now consider how this can be achieved.

USING EXISTING DATA AND TOOLS

One of the software tools used to create a Non Domestic EPC and its recommendation report is the interface to Simplified Building Energy Model (iSBEM) – http://www.ncm.bre.co.uk/.

The Operational Rating Calculation (ORCalc) is the software used to produce the DEC and an advisory report providing advice on energy efficiency measures. (https://www.gov.uk/government/publications/display-energy-certificate-software-specification).

The issue with both rating is that they are designed for compliance purposes and perceived to be for that purpose only.

A POSSIBLE SOLUTION

The tool supporting UK Green Deal is the non-domestic GD tool is based on iSBEM and has the ability to link EPCs and DECs. https://www.ncm-sbem.org.uk/.

The Green Deal tool brings the two assessments together by unlocking the “standard driving conditions” and allowing assessors to tailor the model to real life occupancy. Actual data on how the building is being run and used can now be entered. This allows the asset performance to be compared to the performance in use.

The tool also identifies potential operational measures and quantifies savings from improved management, to the benefit of owners and occupiers. In addition scenarios for asset improvement can be input and the tool calculates the cost / benefits. 

 Dr Andy Lewry is a principal consultant at the Building Research Establishment and author of its new guide, “Bridging the performance gap” – Understanding predicted and actual building operational energy” on which this article is based. The full publication is available from http://www.brebookshop.com/details.jsp?id=327495 quoting reference “PerfGap” for a 20% discount.