content: That is not the only metric, Generally you must consider: Gravimetric density (energy/mass)   volumetric density (energy/volume)   electric potential per cell (ie. voltage)   discharge current (how fast can it discharge) This one is really important and is one of the main reasons we still use lead acid batteries in cars, because they can easily turn over the motor on demand.    Number of cycles possible.  And in this case, as it is a thermal battery, what is the energy loss from conversion of heat to electricity?.... that is an entire field of scientific and engineering research alone.