Energy is a core concept in the teaching of science. Therefore, it is important to know how students' thinking about energy develops so that elementary, middle, and high school students can be appropriately supported in their understanding of energy. This study tests the validity of a proposed theoretical model of students' growth of understanding about energy that moves from a phenomenological understanding, to being able to explain phenomena using basic energy-related concepts, to being able to explain phenomena using more advanced energy-related concepts, often involving atomic/molecular explanations. The study examines results from the administration of 372 distractor-driven, multiple-choice test items aligned to a wide range of energy ideas from energy forms and transformations, to energy transfers, to energy dissipation and degradation, to energy conservation. Over 20,000 students from across the U.S. participated in the study. Rasch modeling provided linear measures of student performance and item difficulty. For most of the 14 targeted energy ideas, an analysis of the item difficulties validated the study's proposed theoretical model of the growth of understanding of the energy concept. Additionally, a cross-sectional analysis of student performance revealed that the high school students outperformed the middle school students and the middle school students outperformed the elementary school students.