Evaluating Operational Energy Performance of Buildings at the Design Stage

There has been a growing awareness for some time that many 'low energy buildings' use more energy than the designers thought they would. As energy costs have risen, this awareness has started to spread to building owners, who hear much about low energy buildings and subscribe to programmes that rate the design of the building, only to find that their 'low energy design' turns out to be have a typical energy bill. The performance of low energy designs is often little better, and sometimes worse, than that of an older building they have replaced, or supplemented.This phenomenon is not restricted to the UK, but has been observed as far afield as the US and Australia. There is a mismatch between the expectations around the performance of new buildings and the reality of the utility bills. This difference between expected and realised energy performance has come to be known as the 'performance gap'.There are two main reasons for this performance gap. The first is that the method of calculating energy use for the purposes of compliance does not take into account all the energy uses in a building. In particular, it does not address energy used by lifts and escalators, for catering facilities, or for server rooms. This energy use can be substantial: in one case study, the National Trust HQ at Swindon, it was found that 60% of the energy use, that for the server room and the catering, was used in just 3% of the floor area, and more than doubled the operational energy use over the design estimates.The second reason for the performance gap is related to site practice. To deliver a building that uses as much energy as expected requires that the design is built as intended, the engineering systems are commissioned effectively and the operators and occupiers of the building understand how to operate and maintain the building so that it delivers the expected performance.This Technical Memorandum (TM) addresses the first reason. It provides building designers and owners with clear guidance on how to evaluate operational energy use more fully, and accurately, at the design stage. After a brief introduction, which explains the need for the guidance in more detail, it explains the importance of making an accurate estimate of the operating hours and likely occupancy of the building. It then sets out how theoperational energy required for the building can be estimated. As well as covering lighting, heating, ventilation and cooling and provision of hot water, it also considers lifts and escalators, small power loads, catering, server rooms and other plant and equipment.In each case, it provides guidance on how to make more accurate estimates based on the intended use and operation of the building. As well as enabling the designer to estimate energy use more accurately, it will highlight any areas where actual energy use may be higher than a typical building, allowing this to be considered at the design stage.This guidance will help to turn low energy designs into low energy buildings that achieve the design energy targets. It is one of several CIBSE actions to promote more effective assessment of energy performance. CIBSE, along with the RIBA and other partners, has been instrumental in the development of the Carbon Buzz website, which provides a practical tool for designers to compare their low energy designs against others on the website, as well as providing benchmarks of expected and realised performance. Users of this guidance will find that Carbon Buzz is based on similar principles and complements this guidance. For more details visit www.carbonbuzz.org.This TM seeks to provide designers with the means to better address operational energy at the design stage. Whilst political leaders in the UK continue to debate the future electricity supply arrangements needed to meet our future energy demands, this guidance offers practical guidance to enable the buildings we are designing, or redesigning, to deliver the energy performance that the owners and operators expect, and to reduce their demands for energy in the years to come.