Storage capacity

Storage capacity

The capacity of an energy storage system indicates the maximum amount of electrical energy it can store. It is one of the key parameters of Battery Energy Storage Systems (BESS) and determines how long energy can be supplied to consumers or the grid.

How it works

Capacity is measured in kilowatt hours (kWh).

It determines how much energy can be stored during a charging process and released during a discharge process.

The higher the capacity, the longer a storage system can supply electricity — regardless of the power output at which the energy is delivered.

The actual usable capacity may differ from the rated capacity, for example due to aging or limitations set by the Battery Management System (BMS).

Typical applications

 • Storage of solar power for use during evening and night hours
 • Backup power supply for several hours or days
 • Energy shifting in industrial applications (load shifting)
 • Combination with dynamic electricity tariffs to optimize costs
 • Provision of balancing services as part of grid services

Key figures

 • Rated capacity (kWh): theoretical maximum storage capacity
 • Usable capacity (kWh): effectively available storage capacity
 • Depth of Discharge (DoD): share of the capacity that can be regularly used
 • Cycle life: number of charge and discharge cycles the storage can perform
 • Aging: decrease of available capacity over the system’s lifetime

Summary

The capacity of a storage system determines how much energy can be bridged over time — for example during periods without generation, grid bottlenecks, or high demand. It is a key factor for the economic performance, self-sufficiency, and security of supply of an energy system.
In combination with power output, capacity provides the complete picture of the operational capability of a Battery Energy Storage System (BESS).