Energy diplomacy is usually conducted by national governments. However, the case of sister city co-operation between the cities of Surabaya, Indonesia, and Kitakyushu, Japan, shows how substate actors can perform energy diplomacy by developing technology to create public spaces that apply energy efficiency and energy-saving principles. This article offers a new angle on energy diplomacy by elaborating on the role of the city government. To future-proof our perspective, we applied qualitative methods by gaining data from in-depth interviews and focus group discussions, then triangulated the result from the literature about energy diplomacy. The article argues that energy diplomacy has expanded in the sense that it is now carried out at municipal as well as national level and yet still adheres to states’ foreign policy agendas in the energy sector. These substate actors ultimately strengthen the principle of energy utilisation that has been regulated at the national level within the framework of bilateral co-operation with other substate actors.