The sustainability index is an adaptive, multicriteria and novel technique that is used to compare different energy storage systems for their sustainability. This innovative concept utilizes both qualitative and quantitative results to measure sustainability through an index based approach. This report aims to compare three different energy storage options for an intermittent renewable energy source. The three energy storage options are lead acid batteries, lithium polymer batteries and fuel cell systems, that are selected due to their availability and the geographical constrain of using other energy storage options. The renewable energy source used is solar photovoltaic (PV). Several technical, economic and environmental factors have been discussed elaborately which would help us to evaluate the merits of the energy storage system for long term storage. Finally, a novel sustainability index has been proposed which quantifies the qualitative and quantitative aspects of the factors discussed, and thus helps us choose the ideal energy storage system for our scenario. A weighted sum approach is used to quantify each factor according to their importance. After a detailed analysis of the three energy storage systems through the sustainability index approach, the most feasible energy storage option was found to be fuel cell systems which can provide a long term energy storage option and also environmental friendly.