In this paper, the energy consumption of M-PSK modulation schemes is investigated under various fading channels. More explicitly, the total energy required to transmit each bit of information is formulated into an objective function. Specifically, Moment Generation Function (MGF) is incorporated into our analysis so that the energy consumption objective function can be shown to be determined by SNR as well as modulation orders. Furthermore, though the optimization of the above-mentioned objective function, optimal modulation orders can be obtained for different fading conditions and various transmission ranges in terms of minimum energy per bit at a target BER. In addition, numerous simulation results under Rayleigh, Nakagami-m and Rician channels were provided.