During the green transformation of the energy sector, the energy storage system plays a vital role in providing flexibility to accommodate renewable energy, thereby ensuring a reliable and low-carbon energy supply. In particular, it can store energy during off-peak periods and release energy during peak periods to support a stable energy supply. Additionally, power consumption patterns are affected by seasonal power demand changes. To accommodate the power fluctuations caused by renewable energy sources (RESs) and dynamic demands of loads in a power system, integrating an energy storage system (ESS) is a promising solution. Combining an energy storage system with fluctuating generators makes it possible to add a controllable aspect to the power system where both power supply and power demands are uncontrollable. This paper proposes a novel, safe, and optimal storage capacity design for reducing the effects of power fluctuations while considering the safe operation of the power system. Here, safe operation means the maximum and minimum capacity limitations of energy storage systems are always preserved in over-charging and over-discharging situations to ensure the safe operation of a power system. A linear programming-based mathematical model is formulated, and the required storage size for each season, i.e., summer, winter, spring, and autumn, is determined using the model developed. To validate the proposed optimization problem, the smart home environment, i.e., iHouse real power generation and consumption data, is employed. Finally, various case studies are designed and analyzed for the optimal capacity sizing of energy storage to guarantee safe operation with renewable sources. The simulation results also show that the proposed storage design can reduce the optimal size of the energy storage even with incremental power load.