With the rapid development of information technology, massive MIMO is becoming attractive for the fifth generation (5G) communication because of its outstanding performance in both spectral efficiency (SE) and energy efficiency (EE). Recently, many algorithms have been proposed to improve the EE while achieving high SE in massive MIMO systems. In previous work, the power amplifier (PA) efficiency is always considered as a constant. However, the PA efficiency changes with the output power in reality. In the practical situation, the simplification which treats the PA efficiency as a constant will not get the EE optimization based on our analysis. In this paper, we propose a more general EE model of massive MIMO systems considering PA efficiency as a variable, and investigate a power allocation algorithm based on zero-forcing (ZF) precoding so that we can guarantee the SE and EE at the same time. Simulation results show the trade-off between EE and SE, demonstrate the distinction with previous work, and imply that relatively higher transmit power will be more energy efficient.