As a part of a larger study of Writing-to-learn (WTL) in introductory physics, we present the effects of the peer review process on student understanding of energy systems. This study looks at student writing from two consecutive semesters, Fall 2018 with 670 students, and Winter 2019 with 603 students. In one of our three WTL activities, students were asked to list the entities that would be included in the energy system of a pumped water storage facility. In our analysis, we looked at both the peer feedback that a student received and the peer work which they read. From the Fall 2018 results, we have found that the students revise their energy systems more often after receiving a peer review comment advising a change to the energy system. We have further found that the written feedback students receive from members of the instructional team directly correlates with how students revise their energy system. However, the analysis of our Winter 2019 data does not support these same conclusions. Our results present the complex, but significant, role that a peer review process plays in building students' understanding of energy systems.