In this work, we present the results of microscopic calculation for the probability of excitation of Er ions by electron-hole pairs confined in Si nanocrystals surrounded by silicon dioxide. For simplicity we consider the case of low pumping of nanocrystals when we may take into account only one electron-hole pair inside of a single nanocrystal. We have found the probability of Er ion excitation in three principally different cases for the location of the Er ion relatively to the nanocrystal: (i) Er ion is situated inside of the nanocrystal, (ii) Er ion is situated at the boundary of the nanocrystal or very close to the boundary, and (iii) Er ion is situated at the considerable distance from the nanocrystal so that the tunneling of electrons and holes may be neglected.