The aims of this cross-grade study were (1) to determine the level of understanding of energy concepts of students at different academic grades and the differences in understanding between these grades and (2) to analyse the conceptual development of these students. Two hundred and forty-three students at 3 different levels (high school, undergraduate, and postgraduate) participated in this study. The students' understandings of energy concepts were determined using a questionnaire, which requested them to define the concept verbally, and to represent it graphically. The most important findings of this study may be summarised as follows. Students from the different groups generally succeeded in defining 'energy' in a similar way, namely as the 'ability to do work'. Nevertheless, some students (including those at university) also provided different alternative conceptions related to the energy concept. In addition, some students also found difficulty in visually analysing the relationships between different variables using graphs. This finding could help explain why attainment levels of all groups falls short in questions that involve the graphical representation of data.