Abstract The biomass yield and energy efficiency of maize and sweet sorghum were evaluated based on the results of an 11-year experiment conducted in a large-area farm in north-eastern Poland. The demand for energy in the production of maize biomass was estimated at 24.4–25.5 GJ ha−1. Energy consumption in the production of sweet sorghum biomass was 2.8 GJ ha−1 lower on average. Mineral fertilization was the major energy input (72–73 %) in the cultivation of both crops, mainly due to the high energy value of mineral fertilizers (66–71 %) and, to a lesser extent, the demand for energy during fertilizer application. The average maize yields in north-eastern Poland reached 21.7 Mg ha−1 dry matter (DM). The biomass yield of sweet sorghum was 4.0 Mg ha−1 DM lower on average. The variability in sweet sorghum biomass yields was nearly 1.5- to 2-fold higher relative to maize. The energy output of maize biomass ranged from 197 to 290 GJ ha−1 y−1, whereas the average energy output of sorghum biomass was 61 GJ ha−1 lower. The energy gain and the energy efficiency ratio of maize biomass were determined at 172−265 GJ ha−1 and 7.7–11.3, respectively. The above parameters were 58 GJ ha−1 and 14 % lower in sweet sorghum biomass, respectively. In north-eastern Poland, sweet sorghum yields exceeded maize yields, and sweet sorghum was characterized by a more favorable energy balance only in years with above-average precipitation.