The thermal time constant distribution of a photovoltaic panel has been measured experimentally. A wide spectrum ranging between a few ms and a few hours have 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 to the observed phenomena. The reason was the top finger shaped contact and the non uniform characteristics of the individual solar cells give rise to a non uniform power distribution and hence a lateral transient heat transfer.