Gravimetric and Volumetric Energy Density
Gravimetric and volumetric energy density describe how much energy a battery stores relative to its mass or volume.
Two different bases
Gravimetric energy density, often called specific energy, is measured in watt-hours per kilogram (Wh/kg). It indicates how much stored energy is carried for a given battery mass and is relevant to vehicle weight, efficiency, payload, and range.
Volumetric energy density is measured in watt-hours per liter (Wh/L). It indicates how much energy fits into a given space and is relevant to battery packaging, cabin space, floor height, and cargo volume. A battery design can perform well on one measure without leading on the other.
The comparison level must also match. Cell-level figures exclude module housings, cooling, electrical connections, structural protection, and control electronics. Module- and pack-level figures include progressively more of this supporting mass and volume, so they are normally lower than the cell figure. Results can also differ if one source uses nominal stored energy and another uses usable energy.
For chemistry, cell construction, and the energy-versus-power trade-off, see Battery Cell Chemistry.