The Breakdown-Voltage-Basic Oil-Stick (BDVB) enables the online determination of the breakdown voltage of transformer oil. This allows valid conclusions to be drawn about the load capacity of the transformer and its remaining service life. The measurement data from the oil stick can be used to determine the optimum time for the next maintenance and significantly reduce operating costs and risks. Compared to existing solutions, the BDVB has unique selling points: it measures more accurately, every second, with considerably less effort and is self-calibrating due to its measuring principle. The sensor is compact. Compared to existing methods, time-consuming sampling and oil analysis in the laboratory, the use of auxiliary materials or annual calibrations can be reduced. The Oil-Stick has a large potential of further variants which, integrated in a Condition-Monitoring-System (CMS), enable a complete transformer monitoring and asset management. In the transformation of the energy system (energy turnaround), the BDVB enables optimal utilization of the transformers as core elements for controlling the power grids on a global level.