Energy is one of the most central and richly connected ideas across all science disciplines. The purpose of this study was to develop a measurement instrument for assessing students' understanding of the energy concept within and across different science disciplines. To achieve this goal, the Inter-Disciplinary Energy concept Assessment (IDEA) was developed through a pilot test, consisting of 49 items presented in pairs: multiple-choice questions followed by open-ended justification questions. The IDEA was administered to 356 college students in the United States. The partial credit Rasch model was applied to establish evidence of validity and reliability of measures for the IDEA. Results showed that the IDEA can produce reliable and valid measures of student understanding of the energy concept in different science disciplines. No statistical difference was found between the average difficulties of items across disciplines. In addition, the four science disciplines were positively correlated in terms of student understanding of the energy concept. When item difficulties were analyzed by science content topics, it was found that students have difficulty in understanding the energy concept in atomic structure, wave, electric and magnetic energy, and modern physics content areas. This result suggests that students' difficulty in understanding the energy concept is compounded with specific science contents.