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Published: 6 May 2024
Contributors: Alice Gomstyn, Alexandra Jonker

What is energy storage?

Energy storage is the capturing and holding of energy in reserve for later use. Energy storage solutions for electricity generation include pumped-hydro storage, batteries, flywheels, compressed-air energy storage, hydrogen storage and thermal energy storage components.

The ability to store energy can reduce the environmental impacts of energy production and consumption (such as the release of greenhouse gas emissions) and facilitate the expansion of clean, renewable energy. For example, electricity storage is critical for the operation of electric vehicles, while thermal energy storage can help organizations reduce their carbon footprints. Large-scale energy storage systems also help utilities meet electricity demand during periods when renewable energy resources are not producing energy. 

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What is the history of energy storage?

One of the most famous inventions designed to store electricity, the battery, dates back to 1800. Italian physicist Andrew Volta used a pile of nickel discs, zinc disks and saltwater-soaked pads to deliver electrical current. Some 60 years later, French physicist Gaston Planté invented a rechargeable battery using lead and sulphuric acid—known as a lead-acid battery. T

hen, in the early 19th century, American inventor Thomas Edison created a different type of rechargeable battery, which used nickel and iron. Canadian chemical engineer Lewis Urry later developed the prototype for the modern alkaline battery in 1957, after researching Edison’s use of zinc.

Two other long-used forms of energy storage are pumped hydro storage and thermal energy storage. Pumped hydro storage, which is a type of hydroelectric energy storage, was used as early as 1890 in Italy and Switzerland before spreading around the world.

Thermal energy storage, or TES, was in use in ice boxes designed for food preservation in the early 19th century. Modern TES systems have helped heat and cool buildings since the early 20th century.

How is the electricity generation capacity of energy storage systems measured?

Electricity generation capacity in energy storage systems can be measured in two ways. Power capacity, or the maximum amount of electricity generated continuously, is measured in watts, such as kilowatts (kW), megawatts (MW) and gigawatts (GW). Energy capacity, or the total amount of energy stored, is measured in watthours, such as kilowatthours (kWh), megawatthours (MWh) and gigawatthours (GWh).

What are the different types of energy storage systems for electricity?

Electrical energy storage (EES) systems commonly support electric grids. Energy storage systems for electric power generation include:

Pumped hydro storage

Pumped hydro storage, also known as pumped-storage hydropower, can be compared to a giant battery consisting of two water reservoirs of differing elevations. The so-called battery “charges” when power is used to pump water from a lower reservoir to a higher reservoir.

The energy storage system “discharges” power when water, pulled by gravity, is released back to the lower-elevation reservoir and passes through a turbine along the way. The movement of water through the turbine generates power that is fed into electric grid systems.

Pumped hydro storage is the most-deployed energy storage technology around the world, according to the International Energy Agency, accounting for 90% of global energy storage in 2020.1 As of May 2023, China leads the world in operational pumped-storage capacity with 50 gigawatts (GW), representing 30% of global capacity.2


While consumers often think of batteries as small cylinders that power their devices, large-scale battery storage installations known as battery energy storage systems (BESS) can rival some pumped hydro storage facilities in power capacity. These electrochemical storage systems vary in composition and can include lead-acid, redox flow, molten salt and lithium-ion batteries.

Lithium-ion batteries currently dominate the market for utility-scale battery storage. As of 2023, the largest lithium-ion battery storage facility in the world was in Monterrey County, California, with a capacity of 550 megawatts.3 Lithium-ion batteries are also used in electric vehicles.


A flywheel is a rotating wheel that stores kinetic energy. Electricity is used to “charge” the wheel by making it spin at high speeds, while the wheel’s rotation at a constant speed stores that energy.

Flywheel energy storage systems (FESS) are considered an efficient energy technology but can discharge electricity for shorter periods of time than other storage methods. While North America currently dominates the global flywheel market—large flywheel energy storage systems can be found in New York, Pennsylvania and Ontario—demand is increasing in Europe.4

Compressed-air energy storage

This energy technology works by using electricity to compress air and store it underground, often in caverns. To generate electricity, the air is released and run through a turbine linked to an electric generator. A handful of CAES plants are operational around the world, including in China, Canada, Germany and the US.

Thermal energy storage at solar power plants

Thermal energy storage (TES) can be found at solar-thermal electric power plants that use concentrating solar power (CSP) systems. Such systems use concentrated sunlight to heat fluid, such as water or molten salt. While steam from the fluid can be used to produce electricity immediately, the fluid can also be stored in tanks for later use.

Hydrogen storage

Electricity can be converted into hydrogen for storage through the electrolysis of water—using electricity to split water molecules into hydrogen and oxygen. The energy is released when hydrogen is used as a fuel for electricity generation, as well as for transportation.


Supercapacitors are electrochemical devices that store energy by collecting electric charges on electrodes (electrical conductors) filled with an electrolyte solution. They can discharge electricity quickly and have long lifecycles.

The U.S. Department of Energy considers supercapacitors underutilized in the power system because of their low energy density, high costs and lack of awareness of their benefits.5 Ongoing innovation in supercapacitor technology could reduce some of their drawbacks and increase their adoption.

What are the different types of thermal energy storage?

The thermal energy storage method used at solar-thermal electric power plants is known as sensible heat storage, in which heat is stored in liquid or solid materials. Two other types of TES are latent heat storage and thermochemical storage. Latent heat storage entails the transfer of heat during a material’s phase change, such as from solid to liquid. Thermochemical storage involves using chemical processes to absorb heat and later release heat.

In addition to its use in solar power plants, thermal energy storage is commonly used for heating and cooling buildings and for hot water. Using thermal energy storage to power heating and air-conditioning systems instead of natural gas and fossil fuel-sourced electricity can help decarbonize buildings as well as save on energy costs.

How does energy storage support renewable energy expansion?

The benefits of energy storage systems for electric grids include the capability to compensate for fluctuating energy supplies: EES systems can hold excess electricity when it’s available and then contribute electricity supply at times when primary energy sources aren’t contributing enough, especially during periods of peak demand.

In addition, EES systems owned by grid customers can provide emergency backup power during grid outages and be integrated into microgridsThe support that energy storage provides to electric grids is considered key in helping the world transition to green energy and achieving a net-zero future.

Energy storage projects can help stabilize power flow by providing energy at times when renewable energy sources aren’t generating electricity—at night, for instance, for solar energy installations with photovoltaic cells, or during calm days when wind turbines don’t spin.

How long can electric energy storage systems supply electricity?

The length of time an EES can supply electricity varies by energy storage project and type. Energy storage systems with short durations supply energy for just a few minutes, while diurnal energy storage supplies energy for hours. Pumped hydro, compressed-air and some battery energy storage systems provide diurnal storage, while other battery systems and flywheels support short duration storage.

Researchers are working on improving energy technologies to allow for electric energy storage systems to supply power for 10 hours or more, which could further stabilize power supplies as more renewable energy sources come online.

The development of such long-duration energy storage (LDES) also has the support of policymakers, with countries such as Spain, the United Kingdom and the US developing plans to encourage LDES projects.

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Resources What is renewable energy?

Renewable energy is energy generated from natural sources that are replenished faster than they are used.

What is a microgrid?

Microgrids are small-scale power grids that operate independently to generate electricity for a localized area, such as a university campus, hospital complex, military base or geographical region.

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Smart grid technology promises to modernize the traditional electrical system.

What is thermal energy?

Thermal energy refers to energy within a system that’s created by the random motion of molecules and atoms.

Renewable energy trends powering a cleaner future

Global capacity for renewable power generation is expanding more quickly than at any time in the last thirty years.

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1Grid-scale Storage.” (link resides outside ibm.com). International Energy Agency, 11 July 2023

2New pumped-storage capacity in China is helping to integrate growing wind and solar power.” (link resides outside ibm.com). Today in Energy. U.S. Energy Information Administration, 9 August 2023.

3Work continues on deconstruction of the old Moss Landing power plant.” (link resides outside ibm.com). Monterrey County Now, 24 November 2023.

4Flywheel Energy Storage Market.” (link resides outside ibm.com). Straits Research, 2022.

5Technology Strategy Assessment: Findings from Storage Innovations 2030 Supercapacitors July 2023.” (link resides outside ibm.com). U.S. Department of Energy, July 2023.