This publication highlights lessons from 26 case studies in the Cook Islands and Tonga. It provides recommendations on improving the implementation of battery energy storage and renewable energy-based hybrid electricity systems.Small island developing states in the Pacific are urgently seeking to address the challenges of climate change, energy security, and energy access by generating more renewable energy and reducing their reliance on imported fossil fuels. The hybrid and battery energy storage implementation recommendations in this publication focus on the technical aspects of robust systems, procurement and contract management, and decision-making processes.