The development of renewable energy is a fundamental measure to resolve environmental pollution and energy crisis, and achieve the carbon peaking and carbon neutrality goals. However, the inherent volatility and fluctuation of renewable energy output bring unprecedented challenges to the planning and operation of the power grid. This paper studies the optimal ratio of renewable energy and energy storage, aiming to minimize power fluctuation. According to the complementary nature of wind and solar resources, the mode of optimal ratio of wind and solar power that leads to minimal power fluctuation is established and is further transformed into linear programming. The optimization problem of energy storage capacity aiming to smooth the renewable energy output is formulated as a multi-parameter linear program, where storage charging power and energy capacities are parameters. The power fluctuation index is expressed as an analytical function in storage parameters, which is convex and piecewise linear. On the basis of the fluctuation index function, the optimal storage capacities can be determined according to the costs. The proposed method provides an illustrative tool and more abundant information for policy and decision-making.