This master thesis investigates the impact of battery energy storage utilization in the ancillary services market on a local electricity grid. Specifically, the study examines the technical impact of battery energy storage installations and how grid operators should handle these types of installations in the future. To achieve this goal, battery energy storage systems simulations using MatLab software based on historical frequency data have been conducted creating an operation scheme for the battery. The battery simulation was applied to the electrical systems consumption and production data, conducting a whole year power flow simulation investigating the effects on peak power demand both for the customer and local grid owner. The worst-case scenarios were also investigated to assure that the grid could withstand the change in production and consumption at any time. The results indicate that such an installation would increase the peak power demand for the local grid owner by a maximum of 12 kW for one week but in general, it would result in a smaller increment and some small decrement. For the customer, the maximum impact is greater both in absolute numbers and in proportion to the peak power demand before with an increment of 16 kW, but in general, there will not be any impact on the effect tariff at all. The results of the worst-case scenario simulations showed that during backup power supply at high load timestamps in combination with FCR-D down there would be contingencies in one cable connection between two nodes, causing the cable current to increase above the rated current. To allow such an installment the local grid owner would need to limit the battery capacity and increase their billings to the customer, expand the dimension of the system or even, increase their effect tariffs to always ensure a stable electricity supply and stable financial balance.