Reliability analysis is usually required as input for feasibility assessments of rainwater harvesting (RWH) systems. This study introduces the concept of elasticity between the volumetric reliability and a cost function to optimize the storage capacity. The elasticity was estimated for three tanks at a campus of the Universidad de Santander, Colombia; by means of a daily water balance under specific conditions of demand (non-potable uses), infrastructure (area and coverage of surface), and climatic variability. This method showed that, the average expected efficiency ranges from 29% to 58% with tanks of 60 m3. These values are 7–10% lower than those expected just from the water balance, but the size of tanks decreases by up to 53%. Therefore, the present method allows not only a comparison between expected efficiencies from average estimations with the optimization from daily water balance, but also with that estimated in conjunction with costs.