The capacitor stores 188MJ and weighs 127.7kg. So that's an energy density of 1.47MJ/kg. One litre of gasoline weighs 0.76kg and gasoline has an energy density of 46.4MJ/kg. Of course they don't say how dense it is, so maybe it has a density of 31.5kg/L, which is what it would take for them to be equal in terms of volume. The capacitor would have to be more than twice as dense as lead to compete on a volume basis. As it is, the energy density of this supercapacitor is comparable to a large number of other advanced batteries and fuel-cell storage systems.