Thermal engineering in buildings has lead to the development of standards to determine the performance of radiators. With the breakthrough of techniques such as cooling ceilings, a slightly different need has appeared: the parameters of characterization have to be independent of the size of the radiator. This study meets this new requirement by modifying the standard two-parameters power law. It is found that the new parameters are also independent of the mass flow that irrigates the panels. Experiments have been performed to confirm this result. We believe that this model is also attractive to other types of isothermal heat sink.