The basic scope of solar collector testing is the determination of the collector efficiency by conducting measurements under specific conditions defined by international standards. The experimental results of testing lead to determination of the parameters of a more or less complex model, usually a 2- or 3-parameter single node steady-state model, which describes the collector behavior. In the present study, a systematic analysis of the contribution of all the uncertainty components on the basis of the ISO 9806-1 test procedure is carried out in order to determine the final uncertainty in the characteristic equation parameters and the instantaneous efficiency of the collector. A step-by-step methodology, based on specific statistical tools, for evaluation of the suitability of the collector models already in use, is proposed. This methodology not only allows an evaluation of the reliability of the testing procedure itself but also a quantification of the goodness-of-fit. Furthermore, if the uncertainty in the characteristic equation parameters is known, the uncertainty in the collector instantaneous efficiency to be predicted can be assessed. This is essential for the reliability of the results of design tools, for which collector efficiency is a key parameter.