A new procedure for testing solar Aat plate collectors under unsteady weather conditions has been developed. It is based on a model which lumps the thermal capacities of the absorber plate, risers, headers and fluid together in one node referenced to the mean temperature of the fluid. The model is derived from a general energy balance concept, and its solution gives the fluid temperature in terms of the collector parameters and its response function. The characteristic parameters defined in the model enable direct comparison with results from steady state tests. The proposed method has been thoroughly validated under different weather conditions ranging from almost steady to highly variable. During the tests, only the fluid flow rate needs to be regulated. Standard programs for non-linear optimisation are used to estimate the collector parameters from experimental data. The results obtained from the proposed method are close to those of the steady state ASHRAE 93-86 standard (within +/-3%); the scatter being within +/-2%, The theoretical predictions of the proposed model are closer to actual measured values when compared to other transient procedures.