Most solar water heating systems do not design to supply all the hot water requirements of load, therefore a conventional kind of heating is required in these systems. The additional heat raise the temperature of water when the temperature drops below the desired output temperature that control system aim to reach. This research work aimed to evaluate the thermal performance of an auxiliary heating system in conjunction with a flat plate solar-assisted water heater system in province of Kurdistan which is located in western part of Iran. The results of this study showed that maximum water temperature in January for Sanandaj was 52°C with no auxiliary heating in some hours of day. Furthermore, , the maximum solar heat gain was 6000 kJ/hr, while the maximum auxiliary heating rate was required in the early morning and late afternoon.
