This thesis examines the performance of residential buildings and the energy systems contained within those buildings by simulating them in the TRaNsient SYstems Simulation (TRNSYS) program. After matching a building's floorplan to that of house local to the College Park area, national and local building surveys were consulted to produce a prototype of the average Maryland home. This home was simulated with ordinary insulation levels, heating, ventilation, and air conditioning (HVAC) equipment, and appliances. Various construction characteristics, including wall insulation, thermostat set points, HVAC equipment, and appliance efficiency were varied to examine the effects of each individual change upon the final annual energy consumption of the building, and in doing so, the value of retrofitting each characteristic was explored. Finally, the most effective energy-saving strategies were combined to model a low-energy home, in order to explore the possibility of refitting an existing home to become a net-zero site energy building. Sensitivity study results were listed, and a net-zero-energy building was successfully simulated.