A straightforward framework for identifying key design parameters and determining their values for a site-built integrated collector storage solar water heater (ICSSWH) under uncertainty is developed. The framework is then used to evaluate the design for a ICSSWH, which uses locally available materials and can be assembled by builders without specialized training in solar system construction. In the framework, first the most important design parameters are identified by a sensitivity analysis using TRNSED software to predict system performance. Then a decision tree is constructed that identifies discrete outcomes of various combinations of design parameters, uncertainty quantities, and resulting system performance as predicted by the TRNSED model. For the ICSSWH that is evaluated in this project, three key design parameters are identified: storage tank size, top surface area, and household daily hot water cosumption. Three different system sizes are evaluated in this manner, a 114-liter, 227-liter, and 341-liter system. Only the 341-liter system has a favorable economic return compared to a conventional, gas heater. All systems present some risk of performing worse than a conventional system. Further research is warranted into higher surface area designs to improve the performance of the system. The expected return of the system selected by this analysis is modestly higher than the return of the option which would be selected without any consideratoin of uncertainty. This indicates that straightforward frameworks, such as this, for considering uncertainty in the design of residential solar water heaters, may offer benefits to builders and home buyers.