Watt-hour per kilogram

The watt-hour per kilogram (unit symbols: W⋅h/kg) is a unit of specific energy commonly used to measure the density of energy in batteries and capacitors.

watt-hour per kilogram
Unit system	Unit accepted for use with SI
Unit of	Specific energy
Symbol	W⋅h/kg
Conversions
1 W⋅h/kg in ...	... is equal to ...
SI units	3600 J/kg

Conversion to SI units

The watt, kilogram, joule, and the second are part of the International System of Units (SI). The hour is not, though it is accepted for use with the SI. Since a watt equals one joule per second and because one hour equals 3600 seconds, one watt-hour per kilogram can be expressed in SI units as 3600 joules per kilogram.

Typical values

As of June 2022, the highest peer reviewed and published results for an electric car battery is an energy density 350 W⋅h/kg, which has achieved 500 cycles with less than 20% capacity fade.^[1] This compares to supercapacitors that are typically rated between 3 and 10 W⋅h/kg,^[2] with the best commercially available supercapacitors as high as 47 W⋅h/kg.^[3]

Nuclear batteries based on betavoltaics can reach up to 3300 W⋅h/kg, although over much longer time periods.^[4]