The use of fossil fuels to generate energy produces greenhouse gases, which is the main factor in climate change. Solar energy is a clean source of energy that is environmentally friendly. The photovoltaic (PV) system is a well-established technology for converting solar energy into electrical energy. Sizing-decision of the PV system is a strategic decision-making process that has a long-term impact on the project. This work proposes an optimization model for the sizing decisions of the grid-connected PV system. The model considers the financial aspects in terms of minimizing project lifetime costs and the non-financial aspects in terms of reducing CO2 emissions and satisfying the planned service level. The decisions are the number of PV modules, the amount of energy purchased from the main grid, and the amount of energy supplied to the main grid. Besides an auxiliary binary variable for an either-or decision, either purchase-from or supply-to the grid. A case study is provided to clarify the model's practicality based on the residential area at King Fahd University of Petroleum & Minerals (KFUPM). It is found that 1,670 PV modules are required to meet the annual demand at an annual cost of M$1.45 over 25 years.