watt-hour per liter
One watt-hour stored in one litre, equal to 3.6 MJ/m³. It is the unit of battery energy density — how much energy a cell delivers per unit of the space it occupies, as distinct from per unit of its mass.
Watch out: Energy density by volume and specific energy by mass are different figures and a cell can be good at one and poor at the other. A cell quoted at 700 Wh/L may sit at 250 Wh/kg, and which number binds depends entirely on whether the design is short of space or short of payload.
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The watt-hour comes from the electrical trade and the litre from the metric system, and their ratio is what a pack designer works in because volume is usually the binding constraint in a vehicle or a phone.
About the watt-hour per liter
A battery has two energy densities and confusing them is the commonest error in comparing cells. Specific energy is watt-hours per kilogram and governs how much the pack weighs. Energy density is watt-hours per litre and governs how much room it takes. Aircraft care about the first; phones care about the second; electric cars care about both and trade them off.
Modern lithium-ion cells run roughly 250 Wh/kg and 700 Wh/L at the cell level. Both figures fall substantially at the pack level once the casing, cooling, busbars and management electronics are counted — pack-level figures are typically 60 to 75 percent of cell-level ones, and a specification that does not say which it means is not telling you much.
The dimension is the same as pressure, which is a genuine curiosity rather than a coincidence: both are energy per volume. But offering a battery energy density in psi would be absurd, so this is its own type.
The same unit does duty elsewhere. Laser powder-bed fusion uses volumetric energy density in J/mm³ to characterise a build recipe, and gas engineering uses BTU per cubic foot or MJ/m³ for heating value — the input to the Wobbe index, which decides whether one gas can substitute for another in an existing burner.
One watt-hour per liter in every energy per volume unit
| 3,600,000 | joule per cubic meter (J/m³) | |
| 1 | watt-hour per liter (Wh/L)this unit | |
| 1 | kilowatt-hour per cubic meter (kWh/m³) | |
| 0.0036 | joule per cubic millimeter (J/mm³) | |
| 3.6 | megajoule per cubic meter (MJ/m³) | |
| 96.6211 | BTU per cubic foot (BTU/ft³) | |
| 0.0163871 | watt-hour per cubic inch (Wh/in³) |