Conventionally, the overall performance rating of a thermosyphon solar water heater considers the thermal performance of the system during the energy-collecting phase and the system cooling loss during the cooling phase. However, this study suggests that the performance rating should also take the heat removal efficiency of the system during the system application phase into consideration. This study modifies the CNS 12557 B7276 test standard and employs a precise, on-line operation to derive the heat removal efficiency of a system. The thermal performance and heat removal efficiency of 12 systems with capacities in the range of 102–446 L are evaluated. An efficiency coefficient, g0, is defined, which represents the synthesis of the characteristic thermal performance, g � , and the characteristic heat removal efficiency, g �. The proposed modified efficiency coefficient is given by g0 ¼ g � � g �, and represents the quasioverall performance of a solar heating system. The coefficient provides an effective measure of the amount of energy provided to the user from a system which collects and stores heat from solar radiation. According to prevailing reg