A developed criterion of the energy efficiency of iron-boron-silicon metallic glasses was implemented in hydrochloric acid solutions. The criterion(1); lim (the conductivity of the metallic glass in aqueous solution to the conductivity of the metallic glass in air) =1 was applied to determine the energy efficiency of the metallic glass in the aqueous solution when the conductivity of a metallic glass in air became equal (decreased) to the steady conductivity of the metallic glass in aqueous solution as a function of time of the exposure of the metallic glass to the aqueous solution. This criterion was used to determine the energy efficiency of metallic glasses with a wide range of hydrochloric acid concentration. The values of the conductivities were determined by the electrochemical impedance spectroscopy (EIS). In addition, the criterion can be applied under diverse test conditions with a predetermined period of the operational life of the metallic glasses as functional materials. Furthermore, variations of the energy efficiency of the metallic glasses as a function of the acid concentration and time were produced by fitting the experimental data to a numerical model using a nonlinear regression method. The profiles of the metallic glasses exhibit a less conservative behavior of the energy efficiency than the applied analytical criterion.