Bio-conversion of different organic waste streams in Hermetia illucens larval biomass suitable for animal feed is a promising sustainable solution across different sectors. However, environmental factors can affect the allocation of energy to fitness, survival and growth of insects in complex ways and can therefore impact the optimal functionality of bio-conversion systems. This study investigated the influence of temperature and diet quality differences on the metabolic rate (MR), development, growth, food intake, and body nutrient composition of H. illucens larvae. The fifth instar larvae were reared on three artificial diets (unbalanced diets: Protein and Carbohydrate, and control diet: Gainesville) and exposed to two constant temperatures (20 and 27 °C), until they reached the prepupa stage. The MR was measured in the sixth instar larvae and the body nutrient composition was measured in the prepupa larvae. The MR of H. illucens larvae was found to increase with temperature, leading to a higher food intake and faster growth rate. When reared on unbalanced diets, the larvae were found to experience metabolic costs and to consequently increase larval development time and decrease food intake. There was a significant temperature × diet interaction across the investigated parameters, indicating that larvae of H. illucens can prioritise energy allocation and adjust their metabolism in order to ensure further development. The nutrient composition reveals that larval nutrient profile can be enhanced through diets. Nonetheless, caution should be taken when exploring this procedure, as it could be associated with an extra metabolic cost.