This paper investigates a power allocation problem for maximizing energy efficiency (EE) in downlink Coordinated Multi-Point (CoMP) systems with non- orthogonal multiple access (NOMA). First, users' achievable data rate and network throughput are analysed under three transmission schemes: 1) all users' signals are jointly transmitted by coordinated base stations (BSs); 2) only cell-edge users' signals are jointly transmitted by coordinated BSs; 3) each user's signals are transmitted by only one BS. Next, we formulate EE maximization problems for the three schemes under the constraints of minimum users' data rate and maximum BS transmit power. The considered problem is non-convex and hard to tackle. To address it, an iterative sub-optimal algorithm is proposed by adopting fractional programming and difference of convex programming. Numerical results show that the near optimality performance of EE can be achieved by using the proposed algorithm with advantages of fast convergence and low complexity. Three transmission schemes of NOMA have superior EE performance compared with conventional orthogonal multiple access scheme in the same CoMP networks.