Policy learning between cities is a vital process to enable the diffusion of more sustainable practices. The Sustainable Development of Energy, Water and Environment Systems (SDEWES) Index provides a composite indicator to benchmark city performance based on 7 dimensions and 35 main indicators. In this research work, the SDEWES Index is applied to 25 cities. Data are collected, normalised, and aggregated to obtain the results. Stockholm (3.29), Espoo (3.25) and Sevilla (2.98) are the top three cities. An average city in the sample receives an index score of 2.75. The paper further develops a benchmarking tool to trigger policy learning based on the index performance. Two typologies of policy learning are discussed to stimulate an exploratory process for catching-up or finding solutions to common needs. On the basis of the total of 56 cities to which the index is applied to date, a search algorithm is developed to match cities in which 10 common patterns are identified. The paper concludes with the prospect of using the SDEWES Index to stimulate innovation for more sustainable cities.