The ultimate minimum energy of switching mechanism for MOS integrated circuits have been studied. This report elucidates the evaluation methods for minimum switching energy of instantaneous discharged mechanism after charging one, namely, recycled energy of the MOS device. Two approaches are implemented to capture this concept. One is a switching energy by the time-dependent gate capacitance (TDGC) model ; the other one by results developed by transient device simulation, which was implemented using Finite Element Method (FEM). It is understood that the non-recycled minimum swhiching energies by both approaches show a good agreement. The recycled energies are then calculated at various sub-micron gate MOS/SOI devices and can be ultra-low power of the MOS integrated circuits, which may be possible to build recycled power circuitry for super energy-saving in the future new MOS LSI. From those results, (1) the TDGC is simultaneously verified by consistent match of the non-recycled minimum switching energies; (2) the recycled switching energy is found to be the ultimate lower bound of power for MOS device; (3) the recycled switching energy can be saved up to around 80% of that of current MOS LSI.