Watt-hour per kilogram
The watt-hour per kilogram is a unit of specific energy commonly used to measure the density of energy in batteries and capacitors. One watt-hour per kilogram is equal to 3,600 joules per kilogram.The batteries that Tesla uses in their electric cars have about 254 W⋅h/kg, compared to supercapacitors that are typically between 3–10 W⋅h/kg, albeit with research ongoing into enabling much higher values.
Nuclear batteries based on betavoltaics can contain up to 3300 W⋅h/kg.
