Non-orthogonal multiple access (NOMA) is considered as a promising technology for improving the spectral efficiency in fifth-generation systems. In this correspondence, we study the benefit of NOMA in enhancing energy efficiency (EE) for a multiuser downlink transmission, wherein the EE is defined as the ratio of the achievable sum rate of the users to the total power consumption. Our goal is to maximize EE subject to a minimum required data rate for each user, which leads to a nonconvex fractional programming problem. To solve it, we first establish the feasible range of the transmitting power that is able to support each user's data rate requirement. Then, we propose an EE-optimal power allocation strategy that maximizes EE. Our numerical results show that NOMA has superior EE performance in comparison with conventional orthogonal multiple access.