<p>Growth rates often increase with temperature, yet organisms generally grow to larger sizes in colder environments. This pattern, referred to as the Temperature-Size Rule, may be better understood by examining how temperature affects both the supply and demand of energy at different body sizes. In this study, we measured the energetics regulating growth in a marine gastropod, <i>Tegula funebralis,</i> by experimentally measuring feeding and respiration across a broad range of sizes and temperatures. This was paired with field measurements of maximum body sizes from 38 populations spanning 18 degrees of latitude. Our experimental results suggest that temperature can have differential effects on the scaling of feeding and respiration rates with body size. Feeding rate increased with body size but at a decelerating rate (scaling exponent = 0.34) and was only mildly affected by temperature. Respiration exhibited a much stronger increase with temperature and shifted from allometric scaling at low temperatures (scaling exponent near 2/3) toward isometric scaling at high temperatures (exponent closer to 1). Energetic scope for growth is thus highly dependent on temperature and driven by the differential sensitivity of energy supply and demand. Our energetic model did a good job explaining temperature-size relationships for <i>T. funebralis</i> (pseudo <i>R</i>² = 0.28), though predictions could be improved appreciably by invoking additional mechanisms (e.g., latitudinal differences in assimilation efficiency or thermal sensitivity), suggesting avenues for future research. Overall, our results suggest that a clearer understanding of the energetics underlying growth can help explain a major life history gradient.</p>