This thesis is the product of an effort to develop a CubeSat Conceptual Design Tool for the California Polytechnic State University CubeSat Laboratory. Such a tool is necessary due to inefficiencies with the current conceptual design process. It is being developed to increase accessibility, reduce design time, and promote good systems engineering within CubeSat development. The development of the architecture of a conceptual design tool, the core user-interface element, and the completion of a module for the electrical power subsystem is the focus of this thesis. The architecture is built around different modules to design different subsystems that work in conjunction. The module in the tool was developed to allow a user to size an electrical power subsystem, and that is the basis for future subsystem development. Model-based Systems Engineering was also utilized as an endpoint for the tool’s outputs, and a CubeSat Model has been built for this effort. Validation has been successful on the Conceptual Design Tool as implemented at this time, so the tool it is ready to design CubeSat electrical power subsystems and be expanded upon by other tool developers.