This paper considers a time-division duplex cell-free massive multiple-input multiple-output downlink with imperfect channel station information and conjugate beamforming scheme. The total energy efficiency is investigated taking into account the hardware power consumption and the backhaul power consumption. We propose an optimal power allocation algorithm which aims to maximize the total energy efficiency, under a per-access point power constraint and a per-user spectral efficiency constraint. This optimization problem can be approximately solved via a sequence of second-order cone programs. Compared with the case of without power control, our proposed power allocation scheme can double the total energy efficiency. Furthermore, we show that, when the number of access points is large, the backhaul power consumption affects significantly the total energy efficiency.