In this paper, we investigate and evaluate the performances of a subcarrier-index modulation (SIM) technique within an orthogonal frequency division multiplexing (OFDM)-based narrow-band (NB)-power line communication (PLC) system. The SIM technique has been proposed and used mainly in wireless communications to enhance energy and spectral efficiencies. To evaluate the advantages of this technique in PLC, Monte Carlo simulations were performed using field measurements of PLC noise and channel frequency response (CFR). The results show significant advantages in terms of improving the overall system energy and spectral efficiencies, especially for single-level modulation. For instance, when using the SIM-OFDM technique, with a binary phase-shift keying (BPSK) modulation, an energy gain of 66.66% and a bit gain of 50%, with respect to the standard modulation, can be observed.