joule per kilogram

Specific energy answers a question that a total energy cannot: is it worth carrying? A litre of diesel and a kilogram of lithium-ion cells both store energy, and the useful comparison is per kilogram — about 45.6 MJ/kg for the diesel and about 0.9 MJ/kg for the cells, a factor of roughly fifty.

That factor is the whole reason electric aircraft are hard and electric cars are not. A car carries its battery on wheels and the weight costs rolling resistance; an aircraft carries it against gravity for the whole flight, and a fuel that burns off gets lighter as it goes while a battery does not.

The conversions worth having: $1\ \mathrm{MJ/kg} = 10^6\ \mathrm{J/kg} = 277.8\ \mathrm{Wh/kg}$, and $1\ \mathrm{Wh/kg} = 3600\ \mathrm{J/kg}$ exactly. The comminution unit is the neat one: $1\ \mathrm{kWh/t} = 3.6\ \mathrm{kJ/kg}$ exactly, because a kilowatt-hour is 3.6 MJ and a tonne is 1000 kg.

One thing this unit cannot tell you is how fast the energy comes out. A battery and a capacitor can hold comparable energy per kilogram and deliver it over hours or over milliseconds — that is specific POWER, a different quantity, and the two trade against each other in almost every storage technology.