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Glossary · Energy industry & energy transition

Energy density

Energy Density storage Battery hydrogen fuel Energy technology

Energy density refers to the amount of energy that is stored, or can be stored, relative to a mass or a volume. It therefore describes how much energy is contained in one kilogram, one litre or one cubic metre of an energy carrier, storage medium or technical storage unit. Common units are kilowatt-hours per kilogram (kWh/kg), megajoules per kilogram (MJ/kg), kilowatt-hours per litre (kWh/L) or kilowatt-hours per cubic metre (kWh/m³). For conversion: 1 kWh equals 3.6 MJ.

Two variants are distinguished. Gravimetric energy density refers to mass, for example kWh/kg. In technical language it is often also called specific energy. Volumetric energy density refers to space, for example kWh/L or kWh/m³. Which quantity matters more depends on the application. In an aircraft or vehicle, every additional kilogram counts. In a building, in a battery storage system on a company site or in a cavern storage facility, by contrast, the available volume may be the limiting factor rather than weight.

Energy density is a technical parameter, but not a complete assessment of a technology. It says nothing about how quickly energy can be stored or withdrawn, how much of it is ultimately usable, what losses occur, how expensive the installation is, how long it lasts or what infrastructure it requires. In the electricity system this distinction is important. Storage units and energy carriers do not act there as isolated substances, but as technical installations with connections, inverters, tanks, safety requirements, operating rules and market roles.

Typical orders of magnitude

Some reference values help with classification. They apply only to the reference level stated in each case and are not system values:

  • Hydrogen: about 120 MJ/kg based on the lower heating value, which is around 33 kWh/kg. Based on the higher heating value (gross calorific value) it is about 144 MJ/kg, or around 40 kWh/kg. This is the highest mass-based value of all common fuels. Under ambient conditions, however, one litre of hydrogen gas contains only about 10.7 kJ, i.e. around 3 Wh/L or about 3 kWh/m³.

  • Liquid hydrogen at about minus 253 °C: around 8.4 MJ/L, which is about 2.3 kWh/L. Petrol (gasoline) is at about 44 MJ/kg (around 12 kWh/kg) and about 31 MJ/L (around 8.6 kWh/L). Per kilogram, hydrogen thus has almost three times the energy of petrol. Per litre, even liquid hydrogen has only a little over a quarter.

  • Lithium-ion battery cells: According to the IEA, the latest generation of lithium iron phosphate (LFP) cells reaches up to about 205 Wh/kg, NMC cells up to about 265 Wh/kg and current sodium-ion cells up to about 175 Wh/kg. Even the best cells thus store per kilogram only around one forty-fifth to one sixtieth of the chemical energy of petrol. At pack or system level the value is lower still.

For hydrogen and other fuels it must be stated whether the value is based on the lower heating value or the gross calorific value. For hydrogen the difference is around 20 percent. If the two bases are mixed, apparent contradictions arise between sources.

Substance, cell, system: the reference level

A common mistake is to equate the energy density of a substance directly with the energy density of a technical system. Petrol, diesel, hydrogen, methane or battery cells can indeed be compared as energy carriers. In practice, however, what counts is how much usable energy the entire system provides. A battery includes cell housings, cooling, power electronics, battery management and safety clearances. Hydrogen includes a pressure tank, compressor, valves, safety zones and, where applicable, a fuel cell or turbine. For liquid fuels, the tank, engine and exhaust treatment belong to the usage side.

Hydrogen shows the difference particularly clearly. At normal pressure the gas has very little energy per volume. Only compression, liquefaction or chemical bonding increase the volumetric energy density, and each of these steps costs energy, technology and money. An example from vehicle tanks: for a range of around 480 kilometres (300 miles), a fuel cell car needs about 5 kg of hydrogen according to the US Department of Energy. A 700-bar storage system for this has a volume of about 200 litres, which is three to four times a typical petrol tank. For such 700-bar systems, the Department gives projected system-level values of about 1.4 kWh/kg and 0.8 kWh/L. This is a fraction of the substance values of around 33 kWh/kg.

For batteries, too, the reference level must be stated. A single cell has a higher energy density than a module or a complete battery system with housing, cooling and electronics. How large this gap is depends strongly on the design. For electric vehicles, cell chemistry is important. Range, charging power, safety and service life, however, result only from the interplay of cells, temperature management, power electronics and operating strategy. For stationary storage, a lower energy density is often acceptable if costs, cycle stability, fire protection, maintenance and availability of sites turn out more favourable. According to the IEA, in 2025 more than 90 percent of newly installed stationary battery storage worldwide used LFP batteries, i.e. the less energy-dense chemistry.

Distinction from power, capacity and efficiency

Energy density describes a stored amount of energy per mass or volume. It is not the same as power. Power indicates how quickly energy is supplied or absorbed, for example in kilowatts or megawatts. Power per mass or volume is called power density. A storage unit can have a high energy density and still be charged or discharged only slowly. Conversely, a storage unit can deliver very high power, but only for a short time. Supercapacitors are an example.

Storage capacity is also not identical to energy density. Capacity indicates how much energy a storage unit can hold in total. A large storage unit with low energy density can contain more energy than a small one with high energy density. For the electricity system this is central. Short-term grid stabilisation, daily shifting of solar power and seasonal storage place very different demands on power, capacity and location.

Efficiency supplements energy density with the question of how much of the energy input can later be used again. An energy carrier with high energy density can perform poorly in the overall system if production, transport, storage and reconversion to electricity cause high losses. Synthetic fuels and hydrogen derivatives can be valuable for certain applications, for example where direct electrification is technically difficult. In many applications close to electricity, however, they compete with battery storage, direct use of electricity or load shifting, which require fewer conversion steps.

Significance in the electricity system

In the electricity system, energy density becomes relevant as soon as electricity is to be shifted in time or space. Wind and solar power are not always generated when it is consumed. Storage, flexible demand, grid expansion and dispatchable generation balance out these differences. Energy density helps determine which form of storage is suitable for which duration and location.

Compared with liquid fuels, batteries have a low energy density, but they operate very efficiently during charging and discharging. They are therefore well suited to applications with frequent cycles, such as electric vehicles, home storage, commercial storage or short-term storage close to the grid. Their strength lies less in a maximum amount of energy per kilogram than in high electrical efficiency, fast controllability and modular design.

Hydrogen and the energy carriers produced from it are discussed above all where large amounts of energy are to be stored or transported over longer periods. Underground caverns can take up a lot of energy without the surface area growing to the same extent. In seasonal storage, large geological storage spaces can partly compensate for the low volumetric energy density of the gas. Reconversion to electricity nevertheless remains lossy. The assessment therefore depends not on energy density alone, but on the task in the system: reserve, industrial process, long-term storage, feedstock substitution or fuel.

With electrification, the significance of the term shifts. In heat pumps, electric motors and industrial processes, electricity often replaces fossil energy carriers. As a result, electricity consumption can rise while total energy use falls, because electrical applications are often more efficient. The lower energy density of a battery therefore does not automatically mean a disadvantage. An electric motor uses stored electricity far more efficiently than a combustion engine uses the chemical energy content of a fuel. The gap in usable drive energy is therefore considerably smaller than a pure comparison of substance values suggests.

Typical misinterpretations

A common oversimplification is that high energy density means technical superiority. This equation ignores how the energy is used. Diesel is very energy-dense as an energy carrier, but a combustion engine converts only part of the chemical energy into motion. A battery stores less energy per kilogram, but supplies it to the electric motor with considerably lower conversion losses. For range, costs and energy demand, the combination of storage, drive and operating profile therefore counts.

A second misinterpretation arises when energy density alone is used as an argument against stationary storage. A stationary storage unit does not have to be carried along. Weight is often less important there than the price per stored kilowatt-hour, ageing, safety, permitting feasibility and the question of how many hours or days are to be bridged. A technology with moderate energy density can make sense in stationary operation if it is inexpensive, durable and easy to control.

A third imprecision concerns the system boundary. Figures on energy density can refer to the pure substance, the cell, the tank contents, the module or the entire installation. In addition there are reference conditions such as pressure, temperature and heating value basis. If these details are missing, comparisons are prone to distortion. Especially in political or media debates about batteries, hydrogen, synthetic fuels and long-term storage, seemingly clear rankings arise in this way. On closer inspection they depend on assumptions about pressure, temperature, efficiency, safety clearance and service life.

System costs and infrastructure

Energy density influences costs, but is not the same as economic viability. A high energy density can make transport, storage and range easier. However, it may require expensive tanks, elaborate cooling, high safety requirements or lossy conversion steps. A lower energy density can be acceptable if the storage unit is operated at a fixed location, uses standardised components and achieves many cycles.

For grids and markets it is also important when and how energy is provided. A storage unit with high energy density but slow response does not help with every type of grid bottleneck or with frequency maintenance. A battery storage unit with limited capacity, by contrast, can provide valuable flexibility if it responds quickly and is controlled in a market-based or grid-serving manner. Energy density therefore describes only part of suitability. The practical role results from mode of operation, market rules and connection conditions.

The term energy density shows what physical limits an energy carrier or storage unit brings with it. However, it does not by itself explain which solution makes sense in the electricity system. For that, mass, volume, power, efficiency, costs, service life, safety and infrastructure must be considered together. Used precisely, that is, with a clearly stated reference level and reference condition, energy density helps to assess technologies not by individual values but by the task they are actually meant to fulfil in the energy system.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 19:45

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