The extensive application of rainwater harvesting (RWH) projects is inhibited by the challenge posed by the dry seasons. In a case study of Mnyundo Primary School, Tanzania, the performance of the RWH system was evaluated using a daily water balance model. The methodology is based on defined dry season parameters -no water days (NWDs), rainwater usage ratio (RUR), and water level in local water storages; while the system operational methods involve users adopting either fixed (constant) demand or variable demand (demand varying with respect to available water in the storage tank), throughout the system utilization. Additionally, the cost of installing an RWH system to achieve a substantial reduction of NWDs to zero was calculated. It was established that the existing system cannot achieve zero NWDs under consideration of both operational methods. However, the greater the number of tanks, the lower the NWD, and in the variable demand operational method, better RUR was achieved. For mitigating water shortages in the dry season, the school should adopt RWH in two buildings under the demand scenario (300 <= demand <= 900 L/d, for the respective water levels in the storage tanks), yielding 58% RUR. The performance of the system can be improved by monitoring water levels and adhering to demand guidelines. These are useful strategies for practitioners in water supply.