The main objectives of the Opsys program include the validation of EMGP2-simulation models for various SPTF-SS2 systems, the development of user friendly software packages for system simulation and simplified correlations for system design. The validation has been completed for the SPTF-SS2-Belgium system and is proceeding for the Danish and Dutch SPTF-SS2-configurations, which include different types of storage tanks. From the results obtained until now, no systematic model errors have been detected and it can be stated that the simulations based on reliable sets of system characteristics, agree with experimental results. The observed discrepancies are within the calculated uncertain margins. With respect to the development of simulation programs based on interactive procedures, a complete modular simulation program package is available consisting of two programs and various data files and help files. The system specific simulation program has been extended with various additional features and system configurations. The final version of the simplified correlation program is being developed and a new approach for a simplified method for medium sized solar water heaters is investigated.