Energy Density of Fuels
A kilogram of enriched uranium contains 3.9 million megajoules of recoverable energy. A kilogram of gasoline contains 46. A kilogram of lithium-ion battery contains 0.9. These ratios — six orders of magnitude across — explain most of what's tractable in transport, aviation and power generation.
Key insights
Nuclear is in a separate universe
Fission of one kilogram of U-235 releases 8.2 × 10¹³ joules — equivalent to burning 2,700 tonnes of coal or 2 million litres of gasoline. This is why a nuclear reactor refuels every 18–24 months while a coal plant of equal output consumes a 100-car train of coal every two days. The constraint on nuclear is everything except energy density.
Hydrocarbons dominate where mass matters
Jet fuel at 43 MJ/kg is non-negotiable for long-haul aviation. The best lithium-ion batteries deliver 0.9 MJ/kg at the pack level — 50× less. Electric aviation works for under 200 nautical miles; battery-electric long-haul is physically infeasible with current chemistries. Synthetic kerosene from green hydrogen is the leading candidate for jet decarbonization.
Batteries trade density for cyclability
Lithium-ion is energy-light but charges and discharges thousands of times at high round-trip efficiency (>90%). Hydrogen has 8× the specific energy of Li-ion but loses 30–40% in the round-trip from electricity through electrolysis, compression, transport and fuel cell. For static, daily-cycling storage, batteries win. For long-duration storage and mobile high-power applications, the calculus shifts.
Specific energy by fuel type
Megajoules per kilogram (log scale would be needed for nuclear)
Key Finding: Excluding nuclear: hydrogen leads on per-kg basis; gasoline and diesel cluster near 45; batteries are an order of magnitude below.
Volumetric energy density (MJ/litre)
Energy per unit volume — matters when tanks must fit
Key Finding: Liquid hydrocarbons (gasoline, diesel) lead on a volume basis. Compressed and liquid hydrogen are bulkier than their per-kg ranking suggests.
Methodology & caveats
Specific energy vs volumetric energy
Specific energy (MJ/kg) governs systems where mass is the constraint — aviation, missiles, mobile equipment. Volumetric energy (MJ/litre) governs systems where space is the constraint — automotive tanks, urban vehicles. The two are not interchangeable; hydrogen is excellent on specific energy and poor on volumetric energy.
Lower vs higher heating value
Heating values are measured at LHV (water leaves as vapour) and HHV (water condenses to liquid, releasing latent heat). For most engines and turbines, LHV is the relevant figure; for condensing boilers, HHV is appropriate. The figures here use LHV. The gap is ~10% for hydrocarbons, larger for hydrogen.
System-level penalties
Raw fuel energy density understates the practical penalty of fuel infrastructure. A Li-ion EV pack delivers ~0.9 MJ/kg cell-level but adds ~30% in BMS, structural mass and cooling. A hydrogen vehicle adds compressor, tank and fuel cell mass. Always compare at the system level for a real engineering decision.