Photovoltaic/thermal (PVT) system is utilized in order to meet the electrical and thermal demand using solar energy in Khuznin, city in Qazvin Province, Iran. Although increasing the temperature of PV cells decreases their efficiency, absorbing of thermal energy leads to raise the overall efficiency of PVT system. The main purpose of this study is modeling and optimal sizing of the system, including PVT installation angle, electrical battery, and thermal storage tank for supplying electrical and thermal loads. In this study, annual hourly irradiation is calculated considering climatic conditions and variable parameters. Then, optimal angles of locating system for maximum solar power absorption and extraction are obtained. Thermal and electrical demand of a household consumer is investigated using physical parameters of a practical model and optimization through genetic algorithm considering actual meteorological data of the region. The capacity of the needed battery is obtained by optimization. Also, in order to insure the supply of the demand in severe unpredicted climatic conditions, the connection to the grid as a backup for the system is necessary. Finally, the environmental effect of this system is discussed. In the previous projects, there has not been any feasibility study in the field of off-grid, combined thermal and electrical power production system for household loads and its optimization considering heat and electricity demand which is discussed in this study. (C) 2015 Elsevier Ltd. All rights reserved.