Modern science standards emphasize knowledge-in-use, i.e., connecting scientific practices with content. For knowledge to become usable in knowledge-in-use performances, students need well organized knowledge networks that allow them to activate and connect sets of relevant ideas across contexts, i.e. students need integrated knowledge. We conducted a longitudinal interview study with 30 students in a 7th grade energy unit and used network analysis to investigate students' integrated knowledge, i.e., their knowledge networks. Linking these results with results from knowledge-in-use assessments, we found a strong connection between integrated knowledge and knowledge-in-use about energy. Further, we found evidence that well-connected ideas around the idea of energy transfer were particularly helpful for using energy ideas in the knowledge-in-use assessments. We present network analysis as a valuable extension of existing approaches to investigating students' knowledge networks and the connection between them and knowledge-in-use.