Numerical simulations were carried out in order to study the channel expansion of a low-energy spark in the air after the spark breakdown that resulted in the formation of initial arc channel. The spark-discharge model considers electric circuit parameters, gas-dynamic expansion processes, thermal gas ionization, non-equilibrium chemical reactions, electron heat transfer, and radiation. A specific feature of the model is its ability to simulate a spark channel expansion in the air when only electric circuit parameters (R, L, C) and the discharge gap length are given. The results of the numerical investigation were compared with experimental data based on schlieren images. In this article, the hot gas kernel evolution and plasma state inside the conducting channel are discussed. The efficiency and the rate of energy entering the spark are studied along with the distribution of thermodynamic gas parameters during the spark?s evolution. An extremely high efficiency of the low-energy electrical discharge and a high resistance of the discharge channel compared to other types of discharges were confirmed.