or 6,912 kilojoule (kJ). Listed below are the ultimate energy storage capabilities of various physical or chemical systems ( ignoring external components ),[6] and the mass required to store 6,912 kJ. Principle Type kJ/gram kg Mechanical Flywheel 1 6.91 Mechanical Torsion Spring 0.0003 23,040 Electrochemical Lead Acid Battery 0.1 69.12 Electrochemical