Battery Energy Storage Systems (BESSs) show promise to help renewable energy sources integration onto the grid. These systems are expected to last for a decade or more, but the actual battery degradation under different real world conditions is still largely unknown. In this paper we analyze 3 years of usage of a lithium titanate BESS installed and in operation on an island power system in Hawai'i. The BESS was found to be operational 90% of the time and stored a cumulative 1.5 GWh of energy, which represents more than 5000 equivalent full cycles on the cells. This paper presents a statistical analysis of the BESS usage, develops a representative duty cycle, and provides an initial estimate of BESS degradation. The battery duty cycle was characterized based on 5 parameters: pulses duration, pulses intensity (current), SOC swing range, SOC event ramp rate, and temperature.