What is energy density?
What energy density means — and why it matters
In plain English: Energy density tells you how much energy is packed into a kilogram of fuel. Higher density = more energy per kilogram = lighter vehicles, smaller fuel tanks, longer ranges. This is why an aeroplane runs on jet fuel (very high energy density) and not on batteries (very low energy density) — a battery pack heavy enough to fly a plane across the Atlantic would be far too heavy to get off the ground. It's also why nuclear submarines can stay submerged for months on a fuel pellet the size of a tennis ball, while a diesel submarine must surface every few days to refuel.
Two different measures:
Specific energy (MJ/kg): Energy per unit of mass. The most commonly cited. Determines how heavy a fuel system needs to be. Critical for aviation and transport.
Volumetric energy density (MJ/L): Energy per unit of volume. Determines how large a fuel tank needs to be. Hydrogen has excellent specific energy (142 MJ/kg) but terrible volumetric density (0.01 MJ/L at atmospheric pressure) — because hydrogen molecules are very light but very spread out. Liquid hydrogen (−253°C) has 8.5 MJ/L — better but still much lower than diesel at 35 MJ/L.
Why this matters for the energy transition: Battery electric vehicles have low energy density but are still viable because motors are so much more efficient than combustion engines (~95% vs ~25%). Ships and planes need much higher energy density — which is why they are the hardest transport sectors to electrify.
Specific energy (MJ/kg): Energy per unit of mass. The most commonly cited. Determines how heavy a fuel system needs to be. Critical for aviation and transport.
Volumetric energy density (MJ/L): Energy per unit of volume. Determines how large a fuel tank needs to be. Hydrogen has excellent specific energy (142 MJ/kg) but terrible volumetric density (0.01 MJ/L at atmospheric pressure) — because hydrogen molecules are very light but very spread out. Liquid hydrogen (−253°C) has 8.5 MJ/L — better but still much lower than diesel at 35 MJ/L.
Why this matters for the energy transition: Battery electric vehicles have low energy density but are still viable because motors are so much more efficient than combustion engines (~95% vs ~25%). Ships and planes need much higher energy density — which is why they are the hardest transport sectors to electrify.
Energy density distinguishes between gravimetric specific energy (Wh/kg or MJ/kg) and volumetric energy density (Wh/L or MJ/L). For system-level analysis, the distinction between fuel-only energy density and system energy density is critical — for hydrogen, the tank, pressure vessel, and fuel cell reduce effective system energy density by 5–10×. For lithium-ion, the pack (including BMS, cooling, casing) reduces cell-level energy density (~300 Wh/kg) to pack-level (~200 Wh/kg, ~720 MJ/kg equivalent). Gravimetric energy density of thermochemical fuels is governed by the H:C ratio — hydrogen produces H₂O (mass 18) per 2 × H atom, carbon produces CO₂ (mass 44) per C atom; higher H:C = more energy per kg of combustion products, explaining why methane (CH₄) > petrol (C₈H₁₈) > coal (mostly C). Nuclear fission energy density is fundamentally different in kind: E=mc² with ~0.1% mass-energy conversion at ~200 MeV per fission event per U-235 nucleus (~235 amu), yielding ~8.2×10⁻¹¹ J/event, or ~2.1×10¹⁰ J/g = 21,000 GJ/kg (theoretical); actual heat extraction from enriched fuel in LWRs: ~45 GWd/t HM (gigawatt-days per tonne of heavy metal), equivalent to ~3,900 GJ/kg of fuel ≈ 3,900,000 MJ/kg.
Source: World Nuclear Association · US NRC Reactor Concepts Manual · Lide, D.R. (ed.) CRC Handbook of Chemistry and Physics
The complete comparison
Energy density — every major source, every figure sourced
All values are specific energy (MJ per kilogram of fuel). The bar lengths are proportional on a log scale — using a linear scale would make every bar except uranium invisible. All figures sourced to named institutions below the table. Uncertainty ranges are noted where applicable.
| Energy source | MJ per kg | Relative (log scale) | × vs lithium battery | Source |
|---|---|---|---|---|
| ☢ Uranium-235 (nuclear fission) | 3,900,000 | 4,333,333 × | World Nuclear Assoc. | |
| ⚡ Hydrogen (H₂, lower HHV) | 120 | 133 × | US EIA / NREL | |
| ⚡ Hydrogen (H₂, higher HHV) | 142 | 158 × | US EIA / NREL | |
| 🔵 Natural gas (methane, LHV) | 50 | 56 × | IEA / US EIA | |
| 🛢 Petrol / Gasoline (LHV) | 44 | 49 × | IEA / US EIA | |
| 🛢 Diesel / jet fuel (LHV) | 45 | 50 × | IEA / US EIA | |
| 🛢 Crude oil | 46 | 51 × | IEA | |
| 🔵 LPG (propane/butane) | 46–50 | ~51 × | IEA | |
| ⬛ Coal (anthracite) | 32 | 36 × | World Nuclear Assoc. | |
| ⬛ Coal (bituminous) | 24–29 | ~28 × | World Nuclear Assoc. | |
| ⬛ Coal (lignite / brown coal) | 10–15 | ~14 × | World Nuclear Assoc. | |
| 🌿 Wood (dry) | 15–17 | ~18 × | IEA / FAO | |
| 🌿 Wood pellets | 17–19 | ~20 × | IEA / Drax | |
| 🔋 Lithium-ion battery (cell) | 0.72–0.9 | 1× (baseline) | BloombergNEF | |
| 🔋 Li-ion battery (pack level) | 0.5–0.72 | 0.7× (pack losses) | BloombergNEF / NREL | |
| ☀ Solar panels (per kg of panel) | ~0.003–0.01 MJ/kg·day | Not directly comparable | NREL |
Note on the bar chart: Bars use a log scale — each visual step represents an order of magnitude difference. On a linear scale, all bars except uranium would be invisible (uranium is 3.25 million times denser than coal). HHV = Higher Heating Value (includes latent heat of water vapour). LHV = Lower Heating Value (excludes latent heat). LHV is the practically relevant figure for combustion applications.
Why this matters for the energy transition
What these numbers mean in practice
✈ Aviation — why batteries can't fly planes
A transatlantic flight requires approximately 100 tonnes of jet fuel (45 MJ/kg). Replacing this with Li-ion batteries at 0.9 MJ/kg (cell level) would require ~5,000 tonnes of batteries — 50× the fuel weight. Even accounting for electric motor efficiency (3× more efficient than jet engine), you'd need ~1,700 tonnes of batteries. A Boeing 787 weighs only 128 tonnes empty. Aviation remains constrained to liquid fuels or hydrogen for the foreseeable future. Source: IATA · Airbus sustainability reports.
🚗 Cars — why EVs can still work
A petrol car's 60-litre tank (~45 kg fuel, ~2,000 MJ) powers a 500 km range. An EV needs only ~70 kWh = 252 MJ for the same range because electric motors are ~4× more efficient than combustion engines. At 0.9 MJ/kg battery density, this requires ~280 kg of battery cells — close to the ~400–600 kg packs in modern long-range EVs. Energy density matters less for cars because they don't need to carry people far above ground. Source: IEA Global EV Outlook 2024.
🚢 Ships — why green ammonia is being developed
A large container ship carries ~10,000 tonnes of fuel oil for a transoceanic voyage. Switching to batteries would require ~450,000 tonnes of Li-ion cells — impossible. Green ammonia (4.3 kWh/L, ~18 MJ/kg) is being pursued because it has reasonable energy density for shipping, can be stored as a liquid at −33°C, and contains no carbon. Hydrogen (142 MJ/kg) is even better in gravimetric terms but requires −253°C cryogenic storage — challenging onboard. Source: IMO 2023 GHG Strategy.
🏭 Grid storage — why pumped hydro dominates
Grid-scale electricity storage doesn't need high energy density — it stays in one place. Pumped hydro's effective energy density (varies by site) can be as low as 0.001 MJ/kg of water, but it works because water is abundant and cheap. The constraint is geography, not weight. Batteries win for grid storage when fast response, small footprint, or absence of suitable terrain is required. Source: IEA Batteries 2024.
⚛ Nuclear — why submarines use uranium
A US Navy Los Angeles-class submarine carries approximately 100 kg of enriched uranium fuel (~3.9 million MJ) — enough for 25+ years of operation without refuelling. The same energy from diesel would require ~87,000 tonnes — impossible to carry. Nuclear energy density is why military submarines are nuclear-powered and why space probes use plutonium radioisotope thermoelectric generators (RTGs). Source: US NRC · US DOE.
🌿 Biomass — why it needs so much land
Wood's low energy density (15–17 MJ/kg) means large volumes must be transported and burned. Drax Power Station burns approximately 8 million tonnes of wood pellets per year for 3.9 GW. A coal plant of the same size would burn approximately 4 million tonnes of coal. And a nuclear plant of the same size would burn approximately 27 tonnes of uranium fuel. The land required to sustainably grow 8 million tonnes of wood is approximately 2 million hectares — an area comparable to Wales. Source: Drax Group Annual Report 2023.
Volumetric energy density
Energy per litre — the storage problem for hydrogen
Volumetric energy density — MJ per litre
Diesel: 35 MJ/L — the reference liquid fuel
Petrol: 32 MJ/L
LPG (propane, liquid): 25 MJ/L
Liquid hydrogen (−253°C): 8.5 MJ/L — 4× less dense than diesel
Compressed hydrogen (700 bar): 4.9 MJ/L — 7× less than diesel
Hydrogen at atmospheric pressure: 0.01 MJ/L — impractical as a fuel tank
Lithium-ion battery: 2.5 MJ/L (cell level)
Green ammonia (−33°C): 11.5 MJ/L
Hydrogen's excellent mass-specific energy density is offset by its very low volumetric density. This is why hydrogen fuel cell vehicles need either large high-pressure tanks (700 bar) or cryogenic storage, and why green ammonia is being investigated as a hydrogen carrier for shipping. Source: US DOE Hydrogen Program · NREL
Petrol: 32 MJ/L
LPG (propane, liquid): 25 MJ/L
Liquid hydrogen (−253°C): 8.5 MJ/L — 4× less dense than diesel
Compressed hydrogen (700 bar): 4.9 MJ/L — 7× less than diesel
Hydrogen at atmospheric pressure: 0.01 MJ/L — impractical as a fuel tank
Lithium-ion battery: 2.5 MJ/L (cell level)
Green ammonia (−33°C): 11.5 MJ/L
Hydrogen's excellent mass-specific energy density is offset by its very low volumetric density. This is why hydrogen fuel cell vehicles need either large high-pressure tanks (700 bar) or cryogenic storage, and why green ammonia is being investigated as a hydrogen carrier for shipping. Source: US DOE Hydrogen Program · NREL
Why volumetric density matters for hydrogen
Hydrogen's great strength is its mass energy density: 142 MJ/kg — 3× petrol by mass. A fuel cell vehicle running on hydrogen gets approximately 3× more energy per kilogram than a petrol engine — and the fuel cell is ~2.5× more efficient than a combustion engine, making the practical advantage even larger.
The challenge: hydrogen is extremely light and spread out. One kilogram of hydrogen gas at atmospheric pressure occupies approximately 11 cubic metres — a cube over 2 metres on each side. This is why hydrogen vehicles use 700-bar carbon fibre tanks or cryogenic liquid hydrogen storage — both adding weight, cost, and complexity.
Toyota Mirai tank: Two tanks, 700 bar, 142.2 litres total, store 5.6 kg hydrogen = 672 MJ — approximately the same as a 20-litre petrol tank (700 MJ). Source: IEA Future of Hydrogen 2023
The challenge: hydrogen is extremely light and spread out. One kilogram of hydrogen gas at atmospheric pressure occupies approximately 11 cubic metres — a cube over 2 metres on each side. This is why hydrogen vehicles use 700-bar carbon fibre tanks or cryogenic liquid hydrogen storage — both adding weight, cost, and complexity.
Toyota Mirai tank: Two tanks, 700 bar, 142.2 litres total, store 5.6 kg hydrogen = 672 MJ — approximately the same as a 20-litre petrol tank (700 MJ). Source: IEA Future of Hydrogen 2023
Questions
Questions people ask — answered
What has the highest energy density of any fuel?
By a vast margin: uranium. Uranium-235 used in nuclear reactors contains approximately 3,900,000 MJ per kilogram — roughly 8 million times more than petrol and 162,000 times more than the best lithium-ion batteries. This is a fundamental consequence of nuclear physics: fission releases energy from within the atomic nucleus (strong nuclear force), while chemical combustion releases energy from electron rearrangement (electromagnetic force). The energy scales are incomparable. In practical terms: 1 kg of uranium fuel can produce as much electricity as approximately 3 million kg (3,000 tonnes) of coal. Source: World Nuclear Association · US NRC.
Source: World Nuclear Association — Heat Values of Various Fuels · US Nuclear Regulatory Commission
Why is hydrogen's energy density measured differently?
Hydrogen has two energy density values quoted, which causes confusion. HHV (Higher Heating Value) = 142 MJ/kg — includes the energy released when water vapour from combustion condenses. LHV (Lower Heating Value) = 120 MJ/kg — the practical value for most combustion applications, where water exits as vapour. HHV vs LHV differences are smaller for other fuels (petrol: 46 vs 44 MJ/kg) but larger for hydrogen because hydrogen combustion produces a lot of water. Additionally, hydrogen's volumetric density is very low at atmospheric conditions (0.09 kg/m³) — requiring compression (350–700 bar) or liquefaction (−253°C) for practical use. The 142 MJ/kg (HHV) refers to mass, not volume. Source: NREL · IEA Future of Hydrogen 2023.
Source: NREL Hydrogen Data · IEA Future of Hydrogen 2023
How do batteries compare to petrol by weight?
Petrol has approximately 44 MJ/kg (LHV). A lithium-ion battery cell has approximately 0.9 MJ/kg — roughly 49× less energy per kilogram. However, this comparison is incomplete — it ignores drivetrain efficiency. A petrol engine converts approximately 20–25% of fuel energy to motion; an electric motor converts approximately 90–95%. This 3–4× efficiency advantage means that in practice, you need about 12–15× more battery weight than petrol weight for the same driving range — not 49×. For a car (where weight is manageable), this works. For a long-haul plane or a container ship (where weight must be minimised), it doesn't. Source: IEA Global EV Outlook 2024 · BloombergNEF.
Source: IEA Global EV Outlook 2024 · BloombergNEF Battery Survey 2023
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Sources
Every source used on this page
World Nuclear Assoc.Heat Values of Various Fuels — uranium, coal, gas, oil, biomassworld-nuclear.org/heat-values
US EIAEnergy content of fuels · Hydrogen and natural gas calorific valueseia.gov/energy-calculators
NRELHydrogen fuel cell data · Vehicle energy comparisonsnrel.gov/hydrogen
IEAFuture of Hydrogen 2023 · Global EV Outlook 2024 · Batteries 2024iea.org/future-of-hydrogen
BloombergNEFBattery price and energy density survey 2023bnef.com/evo
IPCC AR6Sixth Assessment Report · Energy system dataipcc.ch/ar6