We report some of the common, prevalent conceptual resources that students used to reason about energy, based on our analysis of written responses to questions given to 807 introductory physics students. These resources include, for example, associating forms of energy with indicators, relating forces and energy, and representing energy quantitatively. This research responds to a need for large-scale, resources-oriented research on students' conceptual understanding and has the potential to support the development of an underexplored dimension of pedagogical content knowledge-knowledge of student "resources" for understanding energy. Our aim is to promote instructor take-up of the resources theory of knowledge, and we suggest a number of ways in which instructors might capitalize on the resources we report.