Abstract: Measuring progress towards sustainability is an important step in achieving sustainable development but a standard and universally accepted approach does not yet exist. Here, a sustainability assessment methodology for energy systems is developed with the assistance of life-cycle analysis and impact assessment. System-related parameters are compared to target values to yield dimensionless indicators and sub-indicators that are then aggregated into a composite sustainability index using weighting factors. The proposed sustainability assessment methodology is applied to a wind-battery system designed to meet the electrical energy needs of a small community in Southern Ontario. The new sustainability assessment methodology is expected to prove useful as a tool for understanding and fostering sustainable energy systems, alone or in concert with other approaches.