Embodiments of the invention provide techniques and apparatus for accurately estimating an aged capacity (Eaged) of a battery in an energy storage system (ESS) used on a wind farm. Eaged throughout the life of the battery may be estimated by measuring the internal impedance of the battery and using a relationship between the age of the battery (e.g., a number of charging cycles) and a maximum capacity corresponding to the battery. The state of charge (SOC) may be calculated based on the electromotive force (EMF) of the battery. The actual available energy (Eavailable) of the battery may be calculated based on the Eaged of the battery and the calculated SOC. In this manner, embodiments of the present invention may allow a wind farm to function as a virtual power plant (VPP) and deliver peak load electricity, regardless of intermittent wind velocity.