The definition of requirements for complex systems requires a balance between system over-design and associated risk. A severely over-designed system poses minimal risk of failing to meet the user?s operational requirements, however the over-designed system poses increased risk of high costs, usability and reliability due to system complexity, and many other issues. The method proposed in this paper creates a more accurate representation of a complex system in order to determine requirements, and quantifies the risk associated with the defined requirements. The sizing of naval power transformers in electrical distribution systems requires an updated approach to quantify the risk inherent in the system requirements definition process. The proposed methodology uses a systems engineering approach, performing a probabilistic (Monte Carlo) analysis of the electrical loads powered by each, individual transformer. Transformer size can then be determined based on the electrical requirements and the amount of risk the program manager is willing to assume.