In this work, a solar yield prediction tool, named Concept Tool is validated and extended. The tool is built for large-scale solar thermal systems. It is used to calculate the solar yield of collectors, to tailor a system to the requirements of a customer and to estimate the profitability of a planned system. To understand the role of the tool within the company, an extensive survey was conducted with all relevant users. It was the aim to identify areas of improvement and to tailor the tool to the expectations of the user. The main issue for users was the lack of structure and clarity within the tool. Calculations cannot be traced back to the roots easily. Besides that, it lacks the consideration of a storage, of pipe losses and of the incidence angle modifier. The latter has a significant influence on the optical efficiency of collectors and thereby the solar yield. The general input data for the tool, provided by the Meteonorm software, was evaluated first, by comparing it to measured data from the collector test site Puchstraße. It showed that the annual irradiation of Meteonorm compared to measured data is on average 10% higher. The annual average ambient temperature of Meteonorm is approximately 30% lower than measured data. The annual solar yield of four different collector types, measured with heat meters, was compared with the solar yield calculated by the Concept tool, once with Meteonorm data and once with the measured ambient temperature and irradiation. The results displayed, that the Concept Tool’s yield is for both, Meteonorm- and measured data, higher than the measured solar yield. For Meteonorm this could be explained by the higher irradiation. Other possible explanations involved the lack of the incidence angle modifier, faulty collector parameters and the input values of the collector mean temperature. A sensitivity analysis was conducted for the year 2019 to identify the influence of the collector mean temperature, the collector parameters and the limit values of the pump on the yield calculation. The optical efficiency and the collector mean temperature were identified as the main influences on the deviation from the measured yield. As the data, on which the collector mean temperature is based on, is rarely provided more accurately than monthly by the customer and the optical efficiency can be influenced strongly by a soiled front glass plate. The optical efficiency of the collectors was then adapted on an annual basis until the Concept Tool’s solar yield matched the measured solar yield. The degradation over the years, as well as the improvement due to the cleaning of the collectors were visible in a graphic visualization. As the causes of the deviations between the tool and reality were identified, a new Concept Tool was then created, based on the findings of this work.