This paper proposes a methodology for assessment to measure the impact of the inertial frequency response provided by battery energy storage systems (BESS) considering power system uncertainties. The proposed methodology is used to assess the impact of the BESS equipped with sensible frequency controller on the frequency behaviour of the system after a system frequency disturbance considering the variability coming from power system uncertainties. The proposed meteorology is tested in an illustrative power system considering the integration of a grid-scale BESS equipped with inertial frequency response controller, the uncertainty of the power system is simulated by the variability of the power demand. Monte Carlo simulations and time-domain simulations, using DIgSILENT® PowerFactory™ are combined in the implementation of the proposed methodology. Simulation results demonstrate the suitability of the proposed approach to obtain probabilistic distribution functions (PDF) of the main system frequency response.