The STEREO experiment searches for light sterile neutrinos in the eV range. The STEREO detector is divided in two subvolumes both filled with liquid scintillator: an inner neutrino target loaded with gadolinium, which is subsequently subdivided in 6 different cells, and an external unloaded gamma catcher subdivided in 4 cells. The response of each of the cells, including charge to energy conversion and energy non linearities, have been characterized by means of a set of gamma and neutron sources covering a broad range of the energy spectrum of reactor antineutrinos. In addition, the particular response of single photomultiplier tubes and the linearity of electronics are controlled hourly by a system of LEDs performing single photoelectron calibration. This poster will discuss this characterization as a means to obtain an accurate control energy scale and detection efficiency of STEREO, together with the monitoring of the detector’s stability and light cross-talk between cells.