With the constantly increasing installation of PV power and its volatility due to the weather, reliable predictions of the present and future PV electricity feed-in become increasingly important for a stable and cost-efficient electricity supply. In Germany, as in many other countries, only for a small fraction of PV systems the output is monitored in real time. Grid operators have to apply upscaling techniques to estimate the total feed-in by PV systems in their control areas. Currently, these methods mostly rely on power output measurements on a set of reference PV plants. In order to improve the estimation and upscaling of PV power production in the control area of the German transmission system operator TransnetBW we set up a network of 40 solar irradiance measurement stations, measuring and transferring minute resolution irradiance data in near real time These irradiance measurements are used by TransnetBW as an additional high resolution input to their PV power upscaling and forecasting system.