Energy is a scalar physical quantity used to account for the ability of physical systems to produce change and for what is transferred when systems interact.

Energy can be associated with motion, configuration, microscopic structure, fields and other physical states. These descriptions are often called different forms of energy, but energy itself is one conserved quantity rather than a collection of different substances.

The SI unit of energy is the joule ($\mathrm J$).

A system's energy can change because energy crosses its boundary or because the system exchanges energy with its surroundings. Different physical mechanisms of transfer are described more specifically in later models—for example mechanical work, heating or electromagnetic radiation.

The central accounting principle is conservation of energy: energy is not created or destroyed in an isolated system. It can be transferred between systems or transformed between forms while the total remains constant.

For example, when a falling object speeds up, energy associated with the gravitational configuration decreases while kinetic energy increases. When friction slows an object, the mechanical energy does not disappear; energy is transferred into microscopic degrees of freedom of the interacting materials and surroundings.

Energy conservation constrains physical models, but it does not by itself determine how fast a process occurs or which transformations are possible. Those questions require additional laws appropriate to the system being studied.