To a scientist or engineer, the term energy is the potential of a system to perform work or to produce heat. Specific energy is the amount of energy per unit mass. Concentration by mass is the measurement of a quantity per unit mass. Specific energy may be thought of as the concentration of energy per unit mass.
A joule is the metric, or more accurately, the International System of Units (SI) measurement of energy. It is derived from the formula for kinetic energy: Ek = ½ mv2, where m equals the mass of the object in motion and v equals its velocity. A joule is the energy required to accelerate a mass of one kilogram at the rate of one meter per second squared over a distance of one meter. Specific energy is expressed in SI units as joules per gram (J/g), kilojoules per kilogram (kJ/kg), or joules per kilogram (J/kg), depending on the system under study.
An earlier attempt by chemists to define energy was the calorie, the energy required to raise the temperature of one gram of water by 1°C. This is not an exact measurement, however, as the energy required depends slightly on the starting temperature of the water. The SI definition of calorie is now 1 calorie equals 4.184 joules. Specific energy would be measured as calories/gram (cal/g), kilocalories/kg (kcal/kg), or kilocalories per gram (kcal/g).
The British thermal unit (BTU) is the analog to the calorie and is an inexact measurement. A BTU is the energy required to raise the temperature of one pound of water by 1°F. A BTU is slightly less energy than 1 kilojoule. Specific energy in this system is measured as BTUs per pound (BTU/lb).
Light, another form of energy, has no mass. Therefore, no term exists for the specific energy of light. In discussing chemical energy, it is often more useful to be concerned with specific energy on a molecular basis instead of a mass basis. A chemist may use joules per mole (J/mol) instead of joules per kilogram, as it is the number of molecules that react that is useful in this application.