Silvicultural practices that increase wood production in forest plantations are required to achieve society's demand for renewable energy sources. Mixed-species plantations of Eucalyptus and Acacia mangium, a N-fixing species, have been studied as a pathway of ecological intensification, enhancing nitrogen availability and carbon accumulation within the system. Our objective was to compare the potential of bioenergy production in pure and mixed-species plantations. We used calorific values from the literature along with data of wood production measured at five sites in Brazil and Congo with various levels of productivity to calculate the energy produced at the end of stand rotation (on average 6.2 years after planting). We compared pure Eucalyptus stands (100E) to mixed-species stands with the same density of Eucalyptus trees with 25% Acacia trees (100E:25A). At all sites, the mixtures generated energy gains ranging from 0.2 to 36.3% compared to Eucalyptus monocultures. These gains were highly correlated (R2>0.99) with total stemwood production, due to comparable calorific values for both species. This study opens the possibility for using mixed-species plantations of Eucalyptus with N-fixing tree species as a renewable source of bioenergy for both domestic and commercial uses.