During the last decade, insects have received increasing interest as sources of proteins for animal feed and human food. Yellow mealworm (Tenebrio molitor) is considered as one of the best options because of its protein content and its ease of breeding and feeding. This species of insect is commonly reared in crates containing agricultural by-products such as wheat bran. In a mass rearing context, an insect farm can harbor thousands of these crates with different insect life stages. Presently, only little data are available concerning the heat generation of yellow mealworms in rearing conditions, making it hard to scale livestock houses. In this context, the aim of this study was to measure the energy expenditure (EE) of insects at different life stages in optimal rearing conditions (temperature, hygrometry, insect density, water supply and food) during 24 hours using calorimetric cages. We measured feed consumption, body mass, O2 consumption and CO2 production for 10 different insect life stages (i.e eggs, 7mg larvae, 16mg larvae, 29mg larvae, 50mg larvae, 60mg larvae, 79mg larvae, 98mg larvae, pupas and adults). The results describe for the first time the profile of EE for each insect life stage in mass rearing conditions: EE was very low for eggs and quite low for pupas, higher with weight gain for larvae from 7 to 60mg (EE from 0.038 to 0.614 kcal/individual/h), stabilized beyond 60mg body weight (EE about 0.584 kcal/individual/h), and higher in adults (1.033kcal/individual/h). These observations suggest an interesting calorimetric potential for advanced larval stages and adults, in the context of thermal regulation of livestock buildings. Further studies are planned to analyze the complete life cycle of Tenebrio molitor including the impact of the photoperiod. Taken together, these data provide key knowledges for the scaling and sizing of new yellow mealworm farms.