Usable energy is the portion of total energy a system is currently capable of delivering, expressed in megawatt-hours (MWh) and as a percentage of its original capacity. It captures the system’s present performance, accounting for factors like irreversible aging, the presence of weak or degraded cells, imbalances in state of charge, and offline components. As such, it represents both the energy available now and the margin that could potentially be recovered through corrective actions.