This paper discusses and evaluates the quasi-dynamic test procedure for solar thermal collectors according to EN 12975 in view of the uncorrelated regression uncertainty of the collector model parameters obtained from the test. In this discussion the results of two procedures for the regression coefficients identification, the least square (LS) and the weighted least square (WLS) regression methods are analyzed. The uncertainties of the both, the LS and WLS results are calculated and validated. The inter-comparison of the two methods shows comparable normalized efficiency curves, but superior performance of the WLS method with respect to the uncertainties. (c) 2006 Elsevier Ltd. All rights reserved.