Technology basics

The technology used to store energy has important implications for the types of services that can be supplied. Some technologies are only suitable for bulk power applications, whereas others are suitable for both distributed applications and bulk power applications.

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There are a wide variety of energy storage technologies available and operating on the power system today, as well as several novel energy storage technologies that may play a meaningful role in the power system of the future. These technologies are defined by the mechanism or material they use to store electricity. These in turn influence the specific technical characteristics of the energy storage system and the particular applications or grid services suitable for that given technology.


Storage Technology Types and Applications by Storage Medium

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Source: NREL 2020

Technical Characteristics of Energy Storage

Each technology, whether large utility-scale systems like pumped storage hydropower or small behind-the-meter systems like lithium-ion batteries, will have set characteristics and unique advantages and disadvantages that affect the degree to which they are suitable for different applications. The definitions of several of these characteristics are provided below. While some of these characteristics may be unique to a specific technology, many of them broadly apply to all energy storage technologies.


Definitions of Key Energy Storage Technical Characteristics

Technical Characteristic


Rated power capacity

Total possible instantaneous discharge capability (in kilowatts [kW] or megawatts [MW]) of the battery energy storage system (BESS), or the maximum rate of discharge that the BESS can achieve, starting from a fully charged state

Energy capacity

Maximum amount of stored energy (in kilowatt-hours [kWh] or megawatt-hours [MWh]) a battery can hold

Energy/power density

Measure of the energy or power capacity of a battery relative to its volume (kW/L, kWh/L)

Specific energy/power

Measure of the energy or power capacity of a battery relative to its weight (kW/g, kWh/g)

Storage duration

Amount of time storage can discharge at its power capacity before depleting its energy capacity

Cycle life/lifetime

Amount of time or number of cycles a battery storage system can provide regular charging and discharging before failure or significant degradation

Round-trip efficiency

Ratio of the energy charged to the battery to the energy discharged from the battery


Reduction of stored energy of the battery (percentage of charge/time) through internal chemical reactions, rather than through discharging to perform work


The way that the battery is operated, including the times at which it is charged and discharged, and the depth of charge or discharge


The decrease in the battery’s capacity over time and through use

State of charge

The battery capacity as a percentage of its maximum capacity at a given time

Depth of charge

The battery capacity that has been discharged as a percentage of its maximum capacity


Reading List and Case Studies

USAID Energy Storage Decision Guide for Policymakers

National Renewable Energy Laboratory, 2021

As many countries seek to reduce their carbon emissions and increase the amount of VRE on their grids, storage provides an attractive solution to provide grid flexibility and reliability. This report is designed to arm relevant decision makers with the initial layer of information they need to understand energy storage and to make informed policy, regulatory, and investment decisions around grid-connected energy storage. While many of the case studies presented in this report are based on experiences from the U.S. and Europe, the lessons learned can be applied to power sectors in emerging economies. The report is focused on grid-connected storage, meaning storage that is connected to a centralized power system. The USAID Grid-Scale Energy Storage Technologies Primer is a useful companion resource to this report. 

USAID Grid-Scale Energy Storage Technology Primer

National Renewable Energy Laboratory, 2021

Higher penetrations of VRE can drive additional need for power system flexibility. Energy storage is one method of power system flexibility that has gained attention in recent years. This primer is intended to provide regulators and policymakers with an overview of current and emerging energy storage technologies for grid-scale electricity sector applications. This work provides an overview and compares key operating characteristics of select energy storage technologies and explores how these storage technologies may be used to improve the flexibility and reliability of power systems given their technical characteristics. The report covers various electrochemical (lithium-ion, lead-acid, flow, sodium-sulfur), chemical (hydrogen), mechanical (pumped hydro storage, compressed air energy storage, flywheels, gravity) and electrical (supercapacitors) storage mediums. The USAID Energy Storage Decision Guide for Policymakers is a useful companion resource to this report. 

National Renewable Energy Laboratory, 2021

This NREL page provides information on different storage technologies and applications. On this page you can find information on NREL storage research projects as well as recent publications. 

Handbook on Battery Energy Storage System

Asian Development Bank, 2018

This handbook serves as a guide to deploying battery energy storage technologies, specifically for distributed energy resources and flexibility resources. 

DOE/EPRI 2013 Electricity Storage in Collaboration with NRECA

Sandia National Laboratories, Electric Power Research Institute, National Rural Electric Cooperative Association, AECOM Technical Services, Inc, 2013

The Electricity Storage Handbook (Handbook) is a how-to guide for utility and rural cooperative engineers, planners, and decision makers to plan and implement energy storage projects. The Handbook also serves as an information resource for investors and venture capitalists and provides the latest developments in technologies and tools to guide their evaluations of energy storage opportunities.

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