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, assuming a direct link between attitudes and behaviour. Models from psychology highlight a range of additional influential factors and emphasise that occupants do not always act in a rational, linear manner. This paper assesses the impact of various behavioural predictors on small power consumption in offices. 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. 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 predictor: habit. A questionnaire was issued to 44 participants in a UK office. Questionnaire results for each predictor were correlated against each participant’s workstation consumption. Results indicate that occupant interactions with small power appliances are not determined by the conscious process proposed by TPB. Instead they are influenced by automatic decision making, with habit accounting for variations of 19% for workstation consumption. Implications for occupant behaviour models and employee engagement campaigns are discussed.