# Gravimetric energy density

Gravimetric energy density, sometimes referred to as specific energy, is the available energy per unit mass of a substance. Gravimetric energy density is typically expressed in Watt-hours per kilogram (Wh/kg), or Megajoules per kilogram (MJ/kg). The gravimetric energy density gives the energy content of a fuel in terms of storage and handling of the substance. However, gravimetric energy density is not only used for fuels; it can also be used to define battery capacity. This term can be especially useful when talking about materials that need to be stored based on weight, such as batteries in vehicles. The graph on the right shows the energy density (in ) of a variety of different fuels. For example, 1 kg of natural gas has 55 MJ of stored chemical energy. Another example is molecular hydrogen, which has a gravimetric energy density of 120 MJ/kg, which is about 4 times the energy content per mass compared to gasoline. Knowing the gravimetric energy density of raw materials and fuels is important when designing things like batteries. Since batteries need to store energy for long periods of time, using materials that have a greater gravimetric energy storage capacity is essential. Visit the page on energy density for more information about the energy densities of different fuels.

The equation for gravimetric energy density can be seen below.

where:

• is the Gravimetric energy density, an intensive property of materials
• is the available energy (often in megajoules)
• is the mass of the substance (in kilograms)

### Gravimetric Energy Density vs. Energy Density

Gravimetric energy density can be thought of as a more precise definition of energy density. Energy density is used in every day conversations to describe the energy of a substance per volume or per mass. In contrast, gravimetric energy density or specific energy are only used to describe energy per unit mass of a substance.