Energy Storage Energy and Power Capacity

The specifications of any energy storage project generally include power and energy ratings. The power rating, specified here in megawatts (MW), determines the rate of transfer of energy that can be supplied or consumed per unit of time. A system with a higher power rating can charge or discharge quicker than one with a lower power rating. The energy capacity, specified in megawatt-hours (MWh), determines the total amount of energy that the system is able to store or deliver over time. The energy to power ratio (E/P) indicates the time duration (in hours, minutes or seconds) that the system can operate while delivering its rated output. For example, a lithium-ion battery with a power rating of 32MW, and an energy capacity of 8MWh, can deliver power for 15 minutes when discharging at its rated value.

The power and energy requirements may be different based on the target applications. If the primary function of the system is to provide frequency regulation, a higher power rating is preferred so that the system can charge and discharge frequently over a short duration of time. On the other hand, if the primary function is to enable peak-shifting or provide backup power in case of outage, the energy rating will be higher so that the battery can deliver power over a longer time.

Within the context of the optimization algorithm, operation of the energy storage technology is constrained to ensure that its resulting discharge and charge behavior does not occur at a rate exceeding the power capacity defined by the user. Similarly, the user-supplied energy capacity dictates the maximum amount of energy that the system can store when it is fully charged. These values are provided by users in MW and MWh respectively.

The algorithm treats the energy capacity value as usable energy, assuming that the energy storage system can be discharged down to a 0% state of charge and charged to 100%, which may be different than the system’s actual operating parameters. As an example, if the storage system can only be operated between 20% and 100% of its nominal energy capacity, the energy capacity value should be derated to 80% when entered into the tool.

Energy and Power Reference Information

The power and energy specifications that are provided for different energy storage technologies have been obtained from various energy storage projects that have already been or are being implemented. These values serve as practical sample specifications. The links point to the individual references for the different combinations of power (MW) and energy (MWh), represented as (P,E) here, for the different technologies as follows: