Units of energy

Energy is defined via work, so the SI unit of energy is the same as the unit of work – the joule (J), named in honour of James Prescott Joule[1] and his experiments on the mechanical equivalent of heat.

The table illustrates the wide range of magnitudes among conventional units of energy.

Natural gas is often sold in units of energy content or by volume.

Common units for selling by energy content are joules or therms.

In the European Union, food energy labeling in joules is mandatory, often with calories as supplementary information.

In physics and chemistry, it is common to measure energy on the atomic scale in the non-SI, but convenient, units electronvolts (eV).

1 eV is equivalent to the kinetic energy acquired by an electron in passing through a potential difference of 1 volt in a vacuum.

It is common to use the SI magnitude prefixes (e.g. milli-, mega- etc) with electronvolts.

In spectroscopy and related fields it is common to measure energy levels in units of reciprocal centimetres.