| watt-hour per kilogram | |
| Standard: | Unit accepted for use with SI |
| Quantity: | Specific energy |
| Symbol: | W⋅h/kg |
| Units1: | SI units |
| Inunits1: | 3600 J/kg |
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.
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.
The batteries that Tesla uses in their electric cars deliver about 254 W⋅h/kg,[1] compared 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]