A combined system is a new thermal storage strategy adopted in this study with which the two other known strategies namely, partial and full load, are compared. The results revealed that the combined system requires larger equipment size than that required by partial system to satisfy the same cooling load. Factors F-p and F-f that may be multiplied by the daily average cooling load to determine the optimum chillers size for a combined system are found. These factors are applicable for any cooling load and are based on a given chiller condensing and evaporating condition as used in this study. These factors are found to vary with the number of on-peak hours. Combined strategy required chiller size was found to decrease with decrease in on-peak period, hence the optimum chiller size for this new strategy was found to occur at zero on-peak hours, and i.e., when the combined system starts to operate as a partial strategy system.