The energy densitiy is a concept which allows to compare the energy content of e.g. coal, a lithium ion battery, a flying wheel or a hot water tank.
The energy density is measured in Joule per kilogram (gravimetric) or Joule per litre (volumetric). The gravimetric and/or volumetric energy density is used to optimize energy devices for dedicated applications.
In mobile phones or cars both parameters have to be optimized, in the case of a stationary battery the volumetric energy density is much more important than the gravimetric energy density.
Another unit is very common: The energy content in kilowatt hours per kilogram or kilowatt hours per litre.
Typical energy densities are shown in the table below: