This paper presents a method for testing of a grid-size battery energy storage system and its interaction with the grid. The method uses a real installation with relatively small storage capacity which is combined with a simulation model of the grid. The storage is cycled based on measurements from the potential grid location and desired application. A scaling factor is used to relate the physical installation capacity and power ratings with the grid-size storage in the simulation model. Such a hybrid method allows evaluating grid-size storage installations without disturbing the grid. The physical behavior of the battery and control algorithm can be evaluated under realistic conditions without the need to construct a full grid-size storage installation. An experiment is described in this paper were the method is applied to an actual grid and battery storage installation