Ocean thermal energy is one of the sustainable and green energies from ocean, which is from the temperature difference between the warmer surface water and the cooler deep water. Ocean thermal energy conversion (OTEC) plants applies the ocean thermal energy to produce electric power. Unfortunately, the temperature difference in the ocean is not big. Even in the tropical and equatorial regions, the surface water temperature can only reach up to 25°C and the deep water temperature is as low as 4°C. The thermal efficiency of the OTEC plants, therefore, is low. In order to improve the plant thermal efficiency by using the limited ocean temperature gradient, some OTEC plants use the approach of applying multiple stages or adding more equipment for better recovery of the ocean thermal energy, such as heat exchangers and circulating pumps. Obviously, the approach will increase the plant complexity and cost. An important adverse impact of the approach is the additional equipment needs to consume power too, which may affect the plant net power output, in turn the plant thermal efficiency. To address the issue, the author proposes a parameter, plant back work ratio ø, to evaluate if the added equipment is appropriate for the plant power net output and thermal efficiency improvement. In the paper, the author describes varied OTEC plants and conducts the performance calculation of an open-system OTEC plant. The author also illustrates the application of the back work ratio ø as a key parameter on the performance of the OTEC plants in the paper.