In this paper, we address the problem of energyefficient power allocation in MIMO systems. In fact, the widely adopted water-filling power allocation does not ensure the maximization of the energy efficiency (EE). Since the EE maximization is a non-convex problem, numerical methods based on fractional programming were introduced to find the optimal power solutions. In this paper, we present a novel and simple power allocation scheme based on the explicit expressions of the optimal power. We also present a low-complexity algorithm that complements the proposed scheme for low circuit-power regime. Furthermore, we analyze power-constrained and rate-constrained systems and present the corresponding optimal power control. In the numerical results, we show that the presented analytical expressions are accurate and that the algorithm converges within two iterations. We also show that as the number of antenna increases, the system becomes more energy-efficient. Also, a saturation of the EE is observed at high power budget and low minimal rate regimes.