What are Watt-hours per kilogram?

The Watt-hour per kilogram is the reference unit used to measure or indicate the density of energy contained or storable in a body. Its abbreviated form is Wh/kg.

The Watt-hours per kilogram express the energy density as a function of the mass of the body: it is then called mass energy density.

In the same way, it is possible to formulate the energy density according to the volume of the body taken into account: in this case, the energy density is said to be volumetric. The unit used here is the Watt-hour per cubic meter, abbreviated as Wh/m3.


The specifics of Watt-hours per kilogram

The Watt-hour per kilogram (Wh/kg) comes in several variants depending on the magnitude of the energy density taken into account:

  • 1 Kilowatt hour per kilogram (kWh/kg) then corresponds to 1000 Wh/kg ;
  • 1 Megawatt hour per kilogram (MWh/kg) is equivalent to 1000 kWh/kg or 1 million Wh/kg ;
  • 1 Gigawatt hour per kilogram (GWh/kg) is 1000 MWh/kg or 1 million kWh/kg.


Examples and practical application

The mass energy density is an important data in energy production and storage.

The unit Watt-hour per kilogram is widely used in the transportation industry to evaluate the energy contained in a fuel or that can be stored in a battery.


Watt-hours per kilogram in figures

The mass energy density is very variable depending on the material.

Thus, the mass energy density is rather expressed in Wh/kg for:

  • capacitors (1 Wh/kg) and super-capacitors (27.7 Wh/kg) ;
  • alkaline batteries (163 Wh/kg);
  • Lithium batteries (500 Wh/kg).

The kWh/kg is used more for:

  • solid energy sources such as wood (4.5 kWh/kg) and coal (6.6 kWh/kg) ;
  • fuels such as kerosene (11.9 kWh/kg), gasoline (13.1 kWh/kg) and diesel (12.6 kWh/kg).

At the nuclear scale, it is more appropriate to use GWh/kg, with for example:

  • uranium-235 (22 GWh/kg) ;
  • plutonium-239 (23.2 GWh/kg)
  • tritium (93.7 GWh/kg).