Dynamic testing of solar thermal systems is presently defined by ISO 9459-5: 2007 [1]. The testing methodology is clearly defined in the standard. Presently, testing laboratories use DST program [3], a closed source program, for identification of parameters, which is based on a model described in reference [4]. This makes difficult to understand strange results or application to systems with special features. The present paper describes the work done following a different approach for the identification of parameters - use of TRNSYS [5] to simulate the system and use of GENOPT® [6] for optimization. Results are presented and compared with the use of DST for thermosyphon and forced circulation systems. © 2012 The Authors.