C-GEN is a new topology of direct-drive permanent magnet generator being developed at University of Edinburgh. The main benefits are reduced overall system mass and ease of manufacturing, due to the use of an air-cored winding, but with a modular PM rotor consisting of C-core modules. A 15 kW rotary prototype has been fitted and tested on a commercially available wind turbine. Initial sizing studies for wind indicate that the C-GEN concept will be up to 50% lighter than conventional iron cored PM direct drive generators. In addition to wind the C-GEN has applications in marine energy converters. The authors are working with four marine developers to investigate the feasibility of C-GEN for their technology. The results from two of those studies will be presented in this paper namely; Aquamarine Oyster device near shore wave energy converter, Scotrenewables SRTT device tidal current energy converter. In order to find the most optimum solution for the specific application; electromagnetic, thermal, structural and economical design aspects of the generator are defined and coupled using an analytical design tool. To evaluate different design variations an optimization tool is developed based on genetic algorithm. All these considerations make the optimised generator design a very close optimum solution for the "real-world".