Mathematical crop simulation models are useful tools in predicting the potential yield of field crops in a specific environment. The main driving parameter used to estimate biomass accumulation in most of these models is radiation-use efficiency (RUE). Biomass sorghum (Sorghum bicolor L. Moench) is a crop that can be used for energy production (thermal and bioethanol chains) and a knowledge of its RUE in different water supply conditions can help to improve model simulations and evaluate crop diffusion. A 3-year field experiment was carried out in Southern Italy where sorghum was submitted to four irrigated regimes based on actual crop evapotranspiration (ETc). In the first year ETc was measured with weighted lysimeters, while in the other 2 years it was estimated by means of estimated crop coefficient (Kc) and the reference evapotranspiration ET0. The RUE, calculated as the slope of the first-order equation between dry biomass and intercepted photosynthetically active radiation along a crop cycle, showed an average of 2.91 ± 0.54 g MJ–1, even if the RUE proved to be closely correlated with crop water consumption. The latter ranged between 891 and 454 mm and the RUE increased 4.2 mg MJ–1 per mm of water used. A high crop interception of solar radiation was observed in sorghum, reaching its maximum efficiency 40 days after sowing. To obtain high yielding yield biomass sorghum requires a large supply of water, as confirmed by the Kc calculated during the crop cycle, which resulted higher (especially in the development and middle stages) when compared with those reported in the FAO 56 Paper. The obtained RUE values also confirmed a high efficiency in biomass production of this crop, allowing for the introduction of biomass sorghum in the cropping systems of Mediterranean environments as an alternative crop for energy purposes, but with adequate irrigation water supply.