Using TRNSYS simulation program, monthly performance of a PV/T water heater in Malaysia has been simulated and the effect of collector area on the performance of the system has been investigated. In this study, the typical weather year file containing the weather parameters for Kuala Lumpur has been considered. The results indicate that usefulheat gain, electrical power output and water temperature have the maximum values in March, while the minimum values were in July. The results also show that increasing collector area from 1m 2 to 10 m 2 results in an increase in useful heat gain, electrical power output, water temperature and a decrease in auxiliary heating rate. However, when collector area is greater than 7 m 2 , these improvements are small. Therefore it can be concluded that, increasing of collector area from 7m 2 to 10 m 2 has no significant effect on performance of the water heater. Furthermore increasing of heat gain for this range of collector area is negligible. For this PV/T water heater system as the collector area increase from 10 m 2 to 80 m 2 , the monthly useful heat gain, electrical power output and water temperature improves, however, the economic decision, useful heat gain and electrical power output should help in order to select the optimum collector area.
