This paper presents a thermo-economic analysis of an ocean thermal energy conversion (OTEC) power plant for a Nigerian coastal region at latitude 4°00′–4°16′N, and longitude 7°16′–7°19′E, whose energy potential is at 22 and 24 K across 1 km from the sea water surface during rainy and dry seasons, respectively. An ammonia closed cycle OTEC plant was designed to constantly supply 100 MW of electricity all year round to the region. The results of the analysis compared favourably with those for similar plants. The plant’s unit cost of energy was found to be 0.11US$/kWh when compared to 0.1US$/kWh for the municipal energy supply. The break-even point was also found to be 12 years. However, as a result of the high initial installed capital cost of 7954.37US$/kW and life cycle cost of 1.30bUS$, only very large organisations would be able to venture into this project.