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Rising temperatures have important consequences for somatic growth, but observed relationships between temperature and growth can vary in both magnitude and direction. The key to understanding such variation is knowing how temperature affects both the amount of energy available for growth and the efficiency with which surplus energy is assimilated into the body. We tested the hypothesis that patterns of temperature-dependent growth are driven by differential sensitivities of energy intake and expenditure to temperature. Larvae of California grunion Leuresthes tenuis were reared across a range of temperatures and 2 levels of food availability. Energy intake was measured from feeding rate, and energy expenditure was evaluated by measuring respiration and excretion rates. When food was abundant, both intake and expenditure increased with temperature, but intake increased more rapidly. These results suggest that high temperatures should lead to faster growth, and these predictions were confirmed by a separate experiment. In contrast, when food was restricted, the increase in energetic demand with temperature outpaced energy intake, suggesting a dwindling surplus of energy at high temperatures. This predicted reversal of the effects of temperature on growth was also confirmed experimentally. Finally, we compared patterns of energetics and growth to test the effects of temperature on food assimilation efficiency. When food was unlimited, assimilation efficiency decreased rapidly with temperature. When food was restricted, assimilation efficiency remained relatively high. Overall, our results emphasize the value of a bioenergetic perspective for illuminating why and how growth rates are likely to change in a warming ocean.