Objective To investigate nutrition literacy among adult grocery buyers regarding energy-related labelling terms on food packaging. Design Qualitative interviews and quantitative surveys to determine shoppers' understanding of energy terms ('energy', 'calories' and 'kilojoules') and how energy terms affect perceptions of healthiness and intentions to purchase breakfast cereals, muesli bars and frozen meals. Setting Individual in-depth interviews and surveys in two metropolitan supermarkets, Sydney, Australia. Subjects Australian adults (interview n 40, survey n 405) aged 18-79 years. Results The relationship between energy and perceived healthiness of food varied by product type: higher energy breakfast cereals were perceived to be healthier, while lower energy frozen meals were seen as healthier choices. Likewise, intentions to purchase the higher energy product varied according to product type. The primary reason stated for purchasing higher energy products was for sustained energy. Participants from households of lower socio-economic status were significantly more likely to perceive higher energy products as healthier. From the qualitative interviews, participants expressed uncertainty about their understanding of kilojoules, while only 40 % of participants in intercept surveys correctly answered that kilojoules and calories measured the same thing. Conclusions Australian consumers have a poor understanding of energy and kilojoules and tend to perceive higher energy products as healthier and providing sustained energy. This has implications regarding the usefulness of industry front-of-pack labelling initiatives and quick service restaurant menu labelling that provides information on energy content only. Comprehensive and widely communicated education campaigns will be essential to guide consumers towards healthier choices.