Water distribution systems (WDS) and power grids (PG) are critical infrastructure systems that are vital to all human activity. As such, their quality of service is of great importance for economic, environmental, and human welfare reasons. Although traditionally the two systems are analyzed separately, they are interconnected and have mutual effects on one another. WDSs are some of the largest energy consumers, with 7%-8% of the world's total generated energy used for drinking water production and distribution. At the same time, WDS storage facilities allow regulating power loads by load shifting operation policies and even storing energy by using turbines. Therefore, decisions made as part of operating one system influence the operation policy of the other. In order to utilize the potential benefits that the two systems can produce for each other, their design and operation should be analyzed conjunctively. In this paper, a conjunctive optimal design approach for water and power networks is presented, aimed at finding the dimensions of the systems' facilities that will result in minimal overall costs, for both design and operation. The model is formulated, implemented on a simple case study using a nonlinear solver through MATLAB, and analyzed using a comparison to the optimal design of the independent WDS.