Sustainability measurement of a renewable energy system is complex, as is the system itself. The study proposes a general indicator to evaluate sustainability of a renewable energy system. In order to comprehensively assess the environmental, economic and social sustainability, eleven indicators with different dimensions are quantified and aggregated into a general indicator of sustainability (GIS) based on fuzzy decision making methods, such as fuzzy analytic hierarchy processing and fuzzy comprehensive evaluation. The developed GIS is able to provide numerical results of sustainability for a renewable energy system. A case of a renewable energy system in Australia is presented to explain the application of the sustainability indicator. In terms of the four different combinations of grid, solar PV and wind renewable energy, the developed GIS is used to assess their sustainability.