What Is Energy?

The word "energy" appears in nearly every article on the natural sciences — from the reactions inside the Sun to the calories a person burns while exercising. But what is it?

A working definition, and where it falls short

The most familiar phrasing — "energy is the ability to do work" — is the definition the U.S. Energy Information Administration uses for general readers. It is convenient and correct in most everyday situations, but it is not rigorous.

Take a body of gas already in thermal equilibrium with its surroundings: it still carries a great deal of energy, but there is no longer any temperature difference from which to extract work. Under the second law of thermodynamics — as NASA states it — heat flows on its own from a hotter object to a colder one, because only that direction increases total entropy. The energy is all still there; what has run out is the ability to use it.

A sturdier definition is a more modest one: energy is a scalar quantity assigned to a physical system, computable from the state of that system and measured in joules, with one property that earns it a name of its own — in a closed system, that number does not change no matter what happens inside.

This has to be said plainly: physics does not answer the question "what is energy made of". It is not a substance, nor an invisible fluid — it is a number we know how to compute.

The joule: the measure

The SI unit of energy is the joule (symbol J). NIST lists the joule as the derived unit for all three of "energy, work, amount of heat" — three names, one unit, because they are the same thing seen from three sides.

The joule is built from base units: 1 J = 1 N·m = 1 kg·m²·s⁻². One joule is the energy needed to push an object one metre with a force of one newton. Lifting a 100-gram apple one metre takes about 0.98 J.

At either end of the scale other units are used for convenience, but all of them reduce to joules:

• electronvolt (eV) for the particle world: 1 eV = 1.602176634 × 10⁻¹⁹ J. This figure is exact, with no uncertainty, because the elementary charge has had a fixed defined value since 2019 (CODATA 2022).
• kilowatt-hour (kWh) on an electricity bill: one kilowatt for one hour is 1,000 J/s × 3,600 s = 3.6 million J.

Forms of energy

Energy is named according to how it is being held or how it is moving. The grouping below is a convenience for explanation, not a standard classification in physics:

Stored forms. Gravitational potential energy is energy "stored in an object's height" — the higher and heavier the object, the more of it. Chemical energy is energy stored in the bonds between atoms and molecules: food, batteries and fuels are all chemical stores. Nuclear energy is energy stored in the nucleus of an atom — the very thing that holds the nucleus together — and is released when nuclei are fused or split.

Forms of motion. Kinetic energy is the energy of a moving object; NASA gives the familiar expression K = ½mv². Note the exponent: double the speed and the kinetic energy quadruples. Thermal energy is the energy "that comes from atoms and molecules moving in a substance".

Forms in transit. Electrical energy is delivered by tiny charged particles, usually electrons moving through a wire. Radiant energy is electromagnetic energy travelling in transverse waves — this is how sunlight reaches Earth, and it is also the stream of energy that lights up the aurora.

The boundaries between these names are not sharp: thermal energy, in the end, is largely the kinetic energy of the individual particles in a substance.

Conversion, and what stays the same

Energy converts from one form to another; the total does not. NASA puts it compactly: within a defined domain, the amount of energy remains constant — it is neither created nor destroyed.

A familiar chain: nuclear energy in the Sun's core becomes radiant energy, that radiation is captured by plants as chemical energy, chemical energy passes into food, and then becomes the kinetic energy of muscle and heat given off at the skin. Every step changes the name; no step changes the total.

The law of conservation of energy — its precise statement, its scope, and the places it is most often misread — needs an article of its own.

Mass is also a form of energy

The largest extension of the concept comes from special relativity. The U.S. Nuclear Regulatory Commission states the equation E = mc² this way: when the energy of a body changes by an amount E — no matter what form the energy takes — the mass of the body changes by an amount equal to E/c².

Because c² is an enormous number, a very small amount of mass corresponds to a very large amount of energy; that is why nuclear reactions release so much of it. But this is an extension: in everyday phenomena, mass and energy are still accounted for separately without introducing any appreciable error.

A note on naming

"Dark energy" in cosmology is not one of the forms of energy listed above. It is a name given to the unknown cause of the accelerating expansion of the universe — a named gap in our understanding, not a measured store of energy. See dark matter and dark energy.

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Sources: Guide to the SI, ch. 4 and eV-joule relationship — NIST/CODATA 2022; The SI — BIPM; What is energy?, Forms of energy — U.S. EIA; Conservation of Energy, Second Law and Entropy — NASA Glenn; Mass-energy equation — U.S. NRC. Accessed 2026-09-05.