A dynamic method for testing solar flat-plate collectors under unsteady weather conditions has been validated through detailed experiments and compared with two established standards: the ASHRAE 93-86 standard for steady state testing and the British standard BS 6757 for transient testing. The new method is based on a lumped capacity model derived from a general energy balance of the collector under actual conditions. The characteristic parameters are estimated using the standard methods for unconditional nonlinear optimisation. Extensive experiments have been carried out under a wide range of operating and environmental conditions, Four different collectors commercially available in the market have been tested at the same location and using the same experimental rig. The results on the basis of the new method are very close to those obtained from the ASHRAE standard. The average values of F-R(tau alpha)(e) and FRUL by the new method are within +/-3% of the steady state values. The results of the BS 6757 method are within +/-2% for F-R(tau alpha)(e) but those of FRUL are about 12% lower than the ASHRAE values. On average, the difference between the theoretical predictions for the outlet temperature by the new method and the corresponding experimental measurements are about +/-0.3 degrees C, while the predictions by the British standard under the same conditions are about 2 degrees C lower than measured values. The percentage deviations of predictions for the temperature rise based on the two methods, averaged over a day, are about +/-8% and +/-36% respectively. The new dynamic method requires less time for experimentation, one day＇s test is enough to give accurate estimation of the collector parameters. The method does not impose any restriction on the variation of weather or operating parameters and, therefore, has a quite general applicability.