Trade-off decisions, which necessitate striking a balance between two or more desirable but competing features, are a crucial part of design practice. However, they are known to be difficult for student designers to make. While designers, educators, and researchers have numerous methods to assess the quality of design artifacts, these methods are not necessarily easy to use, nor do they indicate design competency. Moreover, they are not grounded in a definition of engineering design. The objectives of this study were twofold. First, we developed a protocol to depict design artifact quality through the lens of design trade-off decisions. We aimed to produce a protocol that: (1) encompasses multiple complementary and competing dimensions, (2) can be applied consistently and systematically, and (3) indicates design competency. We conceptualized a quantitative representation of the degree to which a design artifact addresses human, technical, and economic requirements called the "Trade-off Value Protocol." Second, we tested the "Trade-off Value Protocol" by applying it to 398 middle school students' design artifacts of energy-efficient homes. We used an etic approach of thematic analysis to identify the patterns of variation therein. We found "five distinct patterns of variation" in the set of student design artifacts, which suggested certain trends in the way that students address design dimensions and demonstrate varying levels of design competency. The "Trade-off Value Protocol" isolates an important feature of design competency with which beginning designers often struggle and could be a tool for educators to help students become more informed designers.