Photovoltaic thermal (PVT) systems promise a more optimal solar energy utilization by producing electricity and heat simultaneously. However, the thermal collector design and the way it is connected to the PV panels are still a problem for the PVT system. This study aims to answer the use of thermal paste and insulation in thermal collectors are some of the issues in PVT system manufacturing. An experimental study was conducted by making three scenarios of thermal collectors combined with PV panels and then tested in an indoor solar simulator. Each design was tested with five different cooling water flow rates. This study found that thermal paste and insulation can improve the electrical power of PVT systems by up to 17.59% and its thermal performance increase by 10.13%. Thus, the use of thermal paste and insulation should be done in the manufacturing of each thermal collector for PVT systems.