Photovoltaic Thermal system (PVT) is considered as one of the most important topics to be analyzed these days as a renewable source of energy especially when we are dealing with smart grid. The effect of weather conditions on the Photovoltaic (PV) cell ratio is playing a crucial role on the overall thermal energy (OTE) output of the PVT system and its integration into the grid. In this work we studied the effect of weather conditions on the PV cell ratio to maximize the OTE of the PVT system. Constant and variable flow rate values are applied in the simulated study for different time spans over a year. The proposed work is validated by using three-time span levels: A macro level (annual), meso level (seasonal) and micro level (monthly). Results proved that maximum OTE is reached for a variable PV coverage ratio for PVT system during a year. This study gives recommendations for the percentage of the PV coverage area ratios of the PVT system for each month and each season to maximize the overall system efficiency and minimize CO2 emissions. Different models are proposed in this work and validated for the best PV coverage ratio to generate the optimal OTE. (C) 2018 Elsevier Ltd. All rights reserved.