With the new version T*SOL 2021, the design software for solar thermal systems from Valentin Software has been adapted to the latest guidelines and expanded to reflect market changes. Each component (collectors, pipes, storage tanks, heat generators, etc.) can be parameterized individually from pre-configured solar thermal system variants. Several simulations, both to optimize the system and to verify funding applications, can be carried out simultaneously using the parameter variation, e.g. to automatically dimension collector areas and storage volumes. A variety of tools are available for evaluating the results. The project report with detailed simulation outputs is recognized in Germany as evidence in the context of funding applications. Also, in Austria (Climate and Energy Fund) and Switzerland (Minergie), proof of solar yield and coverage in T*SOL is recognized by official bodies. Graphs with a wide range of temperatures and energy flows can be created for a more in-depth analysis. Anyone who requires an energy label in accordance with EU guidelines can also create this with T*SOL. The extensive databases have been supplemented with additional storage tanks and heat generators, including heat pumps from leading manufacturers. User-defined collectors, storage tanks and boilers can also be created in the program. A comprehensive project report can be created for evaluation with individually definable modules such as system parameters, economic efficiency and simulation results graphics.