When searching for information on energy storage systems and BESS systems for business, terms such as SCE, capacity, power, usable capacity, DOD, efficiency, and EMS are commonly used in technical specifications. These technical terms help explain how much energy a storage system can store, how quickly it can charge or discharge, what portion of its capacity is used in daily operations, and how the entire process is managed.
SCE (Storage Capacity and Efficiency) encompasses two metrics for evaluating a storage system: capacity and efficiency. Capacity is measured in kilowatt-hours (kWh) or megawatt-hours (MWh) and indicates how much energy the storage system can store. Efficiency is expressed as a percentage (%) and indicates the ratio of energy charged into the system to that discharged from it. For example, if 100 kWh is charged into the system and 90 kWh is discharged, the cycle efficiency is 90%. The remaining energy is converted into heat or consumed by inverters, cooling systems, control equipment, and other system components.
Useful capacity is another metric. It indicates the portion of the system's total capacity available for use under normal operating conditions. In technical documentation, the abbreviation DOD (Depth of Discharge) is often used for this. For example, if the system’s total energy capacity is 61.44 kWh and the recommended depth of discharge is 90%, approximately 55.3 kWh is available for normal use. The remaining capacity is reserved for battery protection, stable operation, and a longer service life.
Useful capacity and efficiency should not be confused. Useful capacity indicates how much of the storage device’s capacity can be used, while efficiency refers to how the amount of energy changes during a charge and discharge cycle. In the first case, we are referring to the portion of capacity available for use. In the second, we are referring to the system’s operational process itself.
Power is another important parameter. It is measured in kilowatts (kW) or megawatts (MW) and indicates the rate at which a storage unit can accept or release energy. Two storage units may have the same capacity but different power ratings. One may discharge the same amount of energy faster, while the other may do so more slowly. Therefore, capacity indicates how much energy can be stored, while power indicates how quickly that energy can be delivered.
EMS (Energy Management System) is an energy management system. It determines when to charge and discharge the batteries, and how to coordinate their operation with electricity prices, consumption, and power limits. In a commercial facility, the true value of BESS systems is realised when the batteries do not operate according to default factory settings but, through the EMS, respond to real-time facility data.
Therefore, when selecting a BESS system, one should not focus on a single metric. It is important to evaluate the whole picture: capacity, power, DOD, efficiency, control logic, and the facility's specific needs. These technical metrics are also crucial for the project's financial evaluation: they determine the system’s cost, potential savings, and payback period. Therefore, in addition to technical concepts, it is essential to understand the concepts of energy storage investment and payback.