This paper presents a size optimization method of the energy sources in a standalone photovoltaic system. The proposed technique implies improved photovoltaic array model, dynamic battery model, and accurate objective function as well as a fast simulation algorithm. The loss of load probability (LLP) is used to define the availability of the system. Different system's configurations with different availability levels are generated using the proposed algorithm, These configurations are evaluated based on system availability and cost. Hourly meteorological and load demand data are utilized in this research. A design example is done to show the application of the proposed method considering the weather profile of Malaysia. The result shows that the optimal sizing ratio of the photovoltaic array (C-A) is 1.184, while the sizing ratio for storage battery (C-B) is 0.613. In addition, the levelized cost of energy (LCE) for the unit generated of energy by the proposed system is 0.447 $/kWh. (C) 2017 Elsevier Ltd. All rights reserved.