Energy density
The amount of energy stored per unit mass (specific energy, Wh/kg or MJ/kg) or per unit volume (Wh/L or MJ/L) of a fuel or storage medium.
Energy density comes in two forms: gravimetric (per kilogram, also called specific energy) and volumetric (per litre). Both matter: mass limits aircraft and vehicles, volume limits phones, ships and storage tanks.
Diesel: about 45.6 MJ/kg (12.7 kWh/kg), 38.6 MJ/L (LHV)
Gasoline: about 43.4 MJ/kg (12.1 kWh/kg), 32 MJ/L (LHV)
Hydrogen: 120 MJ/kg (33.3 kWh/kg LHV); 8.5 MJ/L as liquid, about 4.5–5 MJ/L at 700 bar
Li-ion cells: 0.58–1.08 MJ/kg (160–300 Wh/kg)
Pumped hydro: about 0.27 Wh per m³ per metre of head
Liquid hydrocarbons store roughly 40–80 times more energy per kilogram than lithium-ion cells, which explains why aviation and shipping are hard to electrify. The gap narrows when conversion efficiency is counted: an electric drivetrain converts about 85–90% of battery energy into motion, versus 20–35% for internal combustion engines.
Fuel energy density is usually quoted on a lower heating value basis; see calorific value. Compare fuels in the fuel energy content calculator.
Related terms
All terms →- Calorific value (HHV / LHV)
The heat released by complete combustion of a unit quantity of fuel; the higher (gross) value includes condensation heat of water vapour, the lower (net) value excludes it.
- Lithium-ion battery
A rechargeable battery in which lithium ions shuttle between a graphite anode and a metal-oxide or phosphate cathode; the dominant chemistry for EVs and grid storage.
- Kilowatt-hour (kWh)
A unit of energy equal to one kilowatt of power sustained for one hour, or 3.6 megajoules; the standard billing unit for electricity.
- Joule (J)
The SI unit of energy, work and heat: the work done by a force of one newton over one metre, or one watt sustained for one second.
- Green hydrogen
Hydrogen produced by splitting water in an electrolyser powered by renewable electricity, with near-zero direct CO2 emissions.
- LFP (Lithium Iron Phosphate)
A lithium-ion cathode chemistry (LiFePO4) prized for safety, long cycle life and low cost, at the expense of lower energy density than nickel-based cells.