This paper reviews the discrepancy between predicted and measured energy use in non-domestic buildings in a UK context with outlook to global studies. It explains differences between energy performance quantification and classifies this energy performance gap as a difference between compliance and performance modeling with measured energy use. Literary sources are reviewed in order to signify the magnitude between predicted and measured energy use, which is found to deviate by +34% with a SD of 55% based on 62 buildings. It proceeds in describing the underlying causes for the performance gap, existent in all stages of the building life cycle, and identifies the dominant factors to be related to specification uncertainty in modeling, occupant behavior, and poor operational practices having an estimated effect of 20-60, 10-80, and 15-80% on energy use, respectively. Other factors that have a high impact are related to establishing the energy performance target, such as early design decisions, heuristic uncertainty in modeling, and occupant behavior. Finally, action measures and feedback processes in order to reduce the performance gap are discussed, indicating the need for energy in-use legislation, insight into design stage models, accessible energy data, and expansion of research efforts toward building performance in-use in relation to predicted performance. Keywords: energy performance gap, energy use in buildings, predictions, measurements, feedback, postoccupancy evaluation HiGHLiGHTS 1. Classifies the performance gap and analyses its magnitude and underlying causes. 2. The regulatory energy gap is found to deviate by +34% with a SD of 55% based on 62 case study buildings. 3. Specification uncertainty, occupant behavior, and poor practice are dominant underlying causes with an estimated effect of 20-60, 10-80, and 15-80% on energy use, respectively. 4. Action measures to reduce the energy performance gap in contrast to the building life cycle are discussed. 5. There is a need to develop techniques to mitigate the magnitude and underlying causes of the performance gap.

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It is widely accepted that there is a gap between design and real world operational energy consumption. The behaviour of occupants is often cited as an important factor influencing building energy performance. However, its consideration, both during design and operation, is overly simplistic, often assuming a direct link between attitudes and behaviour. Alternative models of decision making from psychology highlight a range of additional influential factors and emphasise that occupants do not always act in a rational manner. Developing a better understanding of occupant decision making could help inform office energy conservation campaigns as well as models of behaviour employed during the design process. This paper assesses the contribution of various behavioural constructs on small power consumption in offices. The method is based upon the Theory of Planned Behaviour (TPB) which assumes that intention is driven by three factors: attitude, subjective norms, and perceived behavioural control, but we also consider a fourth construct: habit measured through the Self-Report Habit Index (SRHI). A questionnaire was issued to 81 participants in two UK offices. Questionnaire results for each behavioural construct were correlated against each participant's individual workstation electricity consumption.The intentional processes proposed by TPB could not account for the observed differences in occupants' interactions with small power appliances. Instead, occupants were interacting with small power "automatically", with habit accounting for 11% of the variation in workstation energy consumption. The implications for occupant behaviour models and employee engagement campaigns are discussed.