Energy is the capacity of a physical system to perform work. Energy exists in several forms such as heat energy, kinetic or mechanical energy, light energy, potential energy, electrical energy, or other forms. According to the law of conservation of energy the total energy of a system remains constant, though energy may transform into another form.
The SI unit of energy is the joule (J) or newton-meter (N-m). Joule is also the SI unit of work.