Fuel cells present an ideal power solution to meet continuous sensory, life-support, and main electrical loads required for operation of unmanned undersea vehicles (UUV) and un-tethered sensors. System concepts for powering a UUV with a solid oxide fuel cell (SOFC) power generator are developed and evaluated through modeling studies of variations in system configurations and design parameters. System concepts that meet the continuous electrical loads for sustained operations and mission selectable capabilities for a 53.3 cm (21-in.) diameter UUV platform with 500 to 1500 W power requirements are analyzed. Two primary system concepts fueled with a liquid hydrocarbon fuel (dodecane) and employing either liquid-oxygen (LOx, Concept I) or a mixture of hydrogen peroxide and water as the cathode gas oxidant (Concept II) are generated and optimized to obtain maximum mission endurance within the volume constraints allocated for the power generator. Concept I (LOx) "best" and "intermediate" designs achieve system efficiencies of 47 and 42.2%-LHV, and mission durations of 90 and 80-h, respectively, at 1000 W power with hull-integrated oxidant storage. The Concept II system manages an efficiency of 45.5%-LHV and a mission duration of 46-h. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3623445] All rights reserved.