In order to compare the domestic hot water heating (DHW) and the power supply performances of a south-facing unglazed flat-plate PVT installed on a roof, with an 8 degrees gradient belonging to a solar house located in Madrid (40.24N, 3.410) with no surrounding obstacles, a finite element heat flux simulation was performed and contrasted with a theoretical mathematical model. In this manner, a correlation between the energy absorbed by water within the pipes in the PVT panel and its inlet temperature, dependant on the tank temperature, is approximated. In both cases, and for certain operating conditions, results are obtained by simulation and by solving the analytical models. The difference between the two methods is used to estimate the degree of freedom introduced. Once the analytical model has been validated by simulation, it is solved for different inlet water temperatures, giving an estimated correlation between the energy absorbed by the Heat Transfer Fluid (HTF) and the solar contribution to DHW. The results show how using this kind of solar panel not only improves the overall performance of a PVT panel with regard to a PV cell performance alone, but it also enables photovoltaic and solar thermal energy to be incorporated in the same area. This is especially crucial when the roof area is limited or restricted, even when the outlet water temperature for DHW is not excessively high. (C) 2016 Elsevier Ltd. All rights reserved.