<p>Organic phase change material and high thermal conductivity filler composites are effective in thermal energy storage, but their performance can be highly dependent on the volume fraction of the phase change material/filler. In these composites, energy density (energy storage capacity of the composite) can be enhanced by increasing the phase change material volume fraction; in contrast, power density (rate at which the energy is accessed) can be improved by increasing the filler volume fraction. However, both characteristics cannot be increased simultaneously; therefore, the optimum volume fraction is crucial for maximizing the composite’s thermal performance. In this work, we leverage the analogy of the thermal energy storage in phase change material with electrochemical energy storage in batteries through the Ragone framework to determine the optimum design. Furthermore, this paper proposes relationships for the optimum volume fraction as a function of thermal load, geometry, and cutoff temperature during the melting process. The proposed correlations provide a practical tool for designing and optimizing organic phase change material-filler composites and reduce reliance on computationally intensive numerical or experimental trials.</p>