Energy is one of the most important unifying themes in science. Yet the way energy is conceptualized varies depending on context. In this paper, the discourse used to explain the role of energy in systems from biology, chemistry, and physics is examined from the perspective of metaphor theory. Six substance metaphors for energy are identified in pedagogical discourse (i.e., textbooks and the science education literature): energy as a substance that can be accounted for, can flow, can be carried, can change forms, can be lost, and can be an ingredient, a product or stored in some way. Each of these conceptual metaphors highlight and obscure various characteristics of energy, and provide a set of frameworks that each afford a different understanding of the energy concept.