In the present thesis, a Power-To-Power system has been experimentally analysed to perform the stack characterization in both SOFC and SOEC operation mode. The aim was to calculate the round-trip efficiency for the stack, defining a performance map able to identify the working zones with higher stack efficiency. In order to achieve such objective, experimental tests in FBK laboratory have been performed with two different strategies: in control voltage and in control current. In case of control voltage, for both the SOFC and SOEC, the stack voltage has been varied with a ramp and the correspondent current values have been observed.