What is "specific energy"

n. (context physics English) The energy of a substance measured per unit mass

Energy density has tables of specific energies of devices and materials.

Specific energy is energy per unit mass. (It is also sometimes called "energy density," though "energy density" more precisely means energy per unit volume.) It is used to quantify, for example, stored heat or other thermodynamic properties of substances such as specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy. It may also be used for the kinetic energy or potential energy of a body. Specific energy is an intensive property, whereas energy and mass are extensive properties.

The SI unit for specific energy is the joule per kilogram (J/kg). Other units still in use in some contexts are the kilocalorie per gram (Cal/g or kcal/g), mostly in food-related topics, watt hours per kilogram in the field of batteries, and the Imperial unit BTU per pound (BTU/lb), in some engineering and applied technical fields.

Kenneth E. Heselton (2004), "Boiler Operator's Handbook". Fairmont Press, 405 pages. ISBN 0881734357

The gray and sievert are specialized measures for specific energy absorbed by body tissues in the form of radiation. Specific strength has the same units as specific energy, and is related to the maximum specific energy of rotation an object can have without flying apart due to centrifugal force. The following table shows the factors for converting to J/kg:

Unit

SI equivalent

kcal/g

4.184 MJ/kg

Wh/kg

3.6 kJ/kg

kWh/kg

3.6 MJ/kg

Btu/lb

2.326 kJ/kg

Btu/lb

ca. 2.32444 kJ/kg

The concept of specific energy is related to but distinct from the chemical notion of molar energy, that is energy per mole of a substance. Although one mole of a substance has a definite molar mass, the mole is technically a non-dimensional unit, a pure number (the number of molecules of the substance being measured, divided by Avogadro constant). Therefore, for molar quantities like molar enthalpy one uses units of energy per mole, such as J/ mol, kJ/mol, or the older (but still widely used) kcal/mol.

Jerzy Leszczynski (2011), "Handbook of Computational Chemistry". Springer, 1430 pages. ISBN 940070710X

For a table giving the specific energy of many different fuels as well as batteries, see the article Energy density.