Energy budget parameters consisting of feed intake, food digestibility, absorption efficiency, oxygen consumption, and ammonia excretion were investigated in 3 size classes of Thai abalone Haliotis asinina (2.22 ± 0.13, 3.27 ± 0.02, and 4.16 ± 0.05 cm in shell length). These abalone were fed with artificial diets (approximately 28% crude protein) and held at 28.0 ± 1.0°C and 31.0 ± 1.0 psu salinity. The daily feeding intake of H. asinina decreased with size class, ranging from 0.75% to 0.48% of their dry weight tissue. However, high absorption efficiencies were observed in all 3 size classes, with an average of 81.04%. Further calculations of the obtained energy from the artificial diet for the 3 size classes were 273.6, 666.9, and 1,744.2 J/day, for the 2-, 3-, and 4-cm size classes, respectively. More than 30% of the obtained energy was used for metabolism (ranging from 33.2–42.5%), whereas the energy used for ammonia excretion was low (ranging from 0.8–1.8%). Scope for growth of the abalone varied between size classes (ranging from 37.4–45.8% of the obtained energy). Results from this study suggest the Thai abalone H. asinina fed with this artificial diet and reared in a semiclosed recirculating water system were in optimal culture conditions, because the feed intake resulted in high-energy absorption that allowed a positive energy balance.