The thermal time constant distribution of a photovoltaic panel has been measured experimentally. A wide spectrum, ranging between a few milliseconds and a few hours, has been detected. A simple thermal analysis can only explain time Constants up to 500 s. It was found that the lateral heat diffusion inside the solar cells was responsible for the observed phenomena. The reason was the top finger-shaped contact and the non-uniform characteristics of the individual solar cells, that gave rise to a non-uniform power distribution and hence a lateral transient heat transfer. The experiment demonstrated that the conventional method used for thermal characterization of semiconductor devices, involving electric preheating, should not be used for photovoltaic panels due to their large dimensions as compared to other electronic devices. (C) 2013 Elsevier B.V. All rights reserved.