<p>Energy Performance Contracting (EPC) is an innovative and useful mechanism for high energy-consumption clients to improve their energy efficiency. However, existing research has not adequately addressed how EPC functions between competitive manufacturers, particularly in coopetition scenarios where firms simultaneously compete and cooperate. In this paper, we investigate the cooperation between two competitive manufacturers under EPC using a game-theoretic model. The model examines a duopoly market where manufacturers engage in Cournot competition while implementing EPC, with the technologically advanced manufacturer acting as the energy service company and the less advanced one as the client. We find that: (i) EPC is an effective path for the high energy-consumption manufacturer lacking energy-saving retrofitting technology and capital to improve energy efficiency; (ii) EPC between two competitive manufacturers in a duopoly market can increase the optimal production quantities and consumer surplus, but obviously reduce the equilibrium price; (iii) Not all EPC is profitable for two competitive manufacturers, and EPC should only be implemented when the market size, the sharing ratio, and the energy-saving retrofit difficulty satisfy specific conditions; (iv) When EPC occurs, two competitive manufacturers will decide the optimal service level and the optimal sharing ratio in EPC individually; (v) EPC effectively reduces total energy consumption and improves environmental performance. These findings provide valuable insights for competitive manufacturers considering EPC implementation and for policymakers seeking to promote energy efficiency in industrial sectors.</p>