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The use of renewable energy resources such as solar and wind energies in today's world is increasing due to environmental problems, reduced fossil fuel resources, and the cost of developing new infrastructure in the current power grid. The most important challenge of using these resources in the power grid is their intermittency characteristic, which using batteries along with these resources can solve this problem to a large extent.This thesis aims to design an optimization method for determining an optimal size and type of battery in a building applied photovoltaic system so that the subscriber's cost is minimized, and the profit is maximized. In addition to these goals, other key performance indicators such as peak shaving, increasing savings, net present value, and self-consumption are also examined, in which the use of batteries improves these key performance indicators. Return on investment and payback period key performance indicators are also involved in selecting the optimal battery. Because the number of key performance indicators is more than one, the fuzzy decision-making method is used to consider the effect of all key performance indicators on optimal battery selection. One of the most important factors that affects the performance of a battery and consequently the final cost is battery degradation. Therefore, in this thesis, this factor is considered in battery modeling.To select the optimal battery according to different Key Performance Indicators, simulation in MATLAB software is done in the form of three cases using real data from Kraftpojkarna Company. The simulation is performed on a building with a 50 kW photovoltaic system capacity located in Stockholm, Sweden. The difference between these three cases is in the type of contract between the subscriber and the utility (Vattenfall company). Based on the simulation results embracing uncertainties in designated boundary conditions, in cases 1 and 2, five in series batteries of BYD HVM 16.6 type with a capacity of 83 kWh and in case 3, one BYD HVM 8.3 battery with a capacity of 8.3 kWh are selected as the optimal battery. The NPVs for cases 1, 2, and 3 are 80000 ± 7000 SEK, 58000 ± 5000 SEK, and 39000 ± 3000 SEK, respectively, indicating that the installation of these batteries is economically justified for the subscriber. The optimal battery installation in cases 1, 2, and 3 reduces the peak load by 9.6 ± 0.9 MW, 9.6 ± 0.9 MW, and 1.6 ± 0.1 MW, respectively. Another Key Performance Indicator that the battery installation improves, is the self-consumption, which in cases 1, 2, and 3, the optimal battery installation increases this KPI by (95.0 ± 8.6) %, (95.0 ± 8.6) %, and (78 ± 7) %, respectively. Finally, it is patent that the results achieved from the studied cases illustrate the effectiveness and aptness of the developed optimization method.