Municipal storage tanks are normally sized according to inherently conservative design guidelines. An alternative way to determine the required size of a tank, on the basis of a stochastic analysis of the system, was proposed in a previous study, in which it was recommended that tanks should be sized for a minimum reliability of one failure in 10 years at the most critical time of the year, typically under seasonal peak demand conditions. In this study, the same method is used to investigate the impact of different user demand parameters on tank reliability. It was concluded that the supply ratio, defined as the ratio of the source capacity to the average demand in the week considered, is the most important demand-related factor affecting tank reliability. It is shown that the reliability of tanks varies greatly throughout the year, and it is recommended that municipalities do everything possible to ensure that their system runs smoothly over the seasonal peak demand period. Several other important demand factors affecting tank reliability are also identified. It is concluded that the optimal combination of source capacity and tank size should be determined on the basis of economic factors, and that it is likely to be system specific.