Energy is a fundamental unifying concept of science, yet common approaches to energy instruction in middle school have shown little success with helping students develop their naive ideas about energy into more sophisticated understandings that are useful for making sense of their experiences. While traditional energy instruction often focuses on simple calculations of energy in idealized systems, we developed a new middle school energy unit that focuses qualitatively on the energy transformations that occur in everyday, nonidealized, systems. In this article, we describe our approach to energy instruction and report the effects this approach had on students' energy conceptions, ability to perform on distal criterion-referenced assessments, and preparation for future energy-related learning. Results indicate that during instruction, students' energy conceptions progress from a set of disconnected ideas toward an integrated understanding that is organized around the principle of transformation, and that these more integrated conceptions both boost students' ability to make sense of everyday phenomena and lay the groundwork for more efficient and meaningful energy-related learning in the future. (Contains 10 tables and 6 figures.)