This study supports the effort of the Association of Southeast Asian Nations (ASEAN) to improve and harmonize air conditioner energy-efficiency standards by adopting cooling seasonal performance factor (CSPF) in accordance with International Organization for Standardization (ISO) Standard 16358. It was undertaken by UNEP U4E and Lawrence Berkeley National Laboratory in consultation with the ASEAN CSPF Project Technical Working Group and provides an overview of seasonal air conditioner energy-efficiency metrics, assesses the regional climatic conditions in ASEAN countries, and makes recommendations for adopting ISO standard 16358 in a harmonized way across the region.