The secure and efficient operation of extra-high voltage grids is a central task of transmission system operators. Accurate grid calculations that account for uncertainties are essential to detect congestion events early. With the substantial expansion of solar and wind generation and the associated weather-data uncertainty, potential congestion events may occur that must be prevented or mitigated. This contribution presents a case study of Germany’s transmission system to quantify and evaluate weather-related uncertainties affecting power flow, taking ensemble-based forecasts into account. The aim is to investigate the effects of ensemble forecasts on the loading of transmission lines and to derive initial insights for grid operation. The freely available SimBench grid model is used and extended with ensemble forecasts at grid node level for solar and wind generation. The study analyzes a full-resolution grid model in an hourly resolution using DC power-flow calculations with the open-source software pandapower. Both probabilistic and deterministic methods are applied to analyze the distribution of line loadings. The results show that the use of ensemble forecasts has a direct impact regarding the possible loadings of the transmission lines within the grid calculation and leads to different spreads of line loadings. These spreads show that the use of ensemble forecasts for wind and solar generation can lead to advantages in grid operation and congestion management. The uncertainties arising from different feed-in values may provide system operators with a better insight into possible congestions.