This paper presents a method to estimate the maximum level of variable energy resources that can be integrated into the grid based on the frequency security constraint. The method described uses the approximation of the frequency deviation extremum based on the sensitivity analysis. The approximated results are then employed to estimate the change of the maximum frequency deviation following the change of inertia and the equivalent regulation constant in the presence of variable energy resources. The frequency security constraint is then used to define the maximum level of variable energy resources penetration. The approximation model has been tested on two test systems and the obtained results are compared with those produced by the simulation. The comparison shows that the proposed method can be utilized to define the maximum level of variable energy resources penetration while securing the stability and reliability of the system without losing notable accuracy.