Thermal performance of the solar thermal systems are estimated using numerical methods and software since the solar processes are transitient in nature been driven by time dependent forcing functions and loads. The system components are defined with mathematical relationships that describe how components function. They are based on first principles (energy balances, mass balances, rate equations and equilibrium relationships) at one extreme or empirical curve fits to operating data from specific machines such as absorption chillers. The component models are programed i.e. they represent written subroutines which are simultaneously solved with the executive program. In this thesis for executive program is chosen TRNSYS containing library with solar thermal system component models. Validation of the TRNSYS components models is performed i.e. the simulation results are compared with experimental measurements. With the simulations are determined the long-term system performance i.e. data are obtained for the energy consumption, solar fraction, collector efficiency also it is performed parametric analysis to determine the influence of specific parameters like collector area, tilt and orientation, mass flow rate etc. to the system performance. In this thesis are considered only the residential buildings.