An apparatus for calibrating a power measurement system for power transformers, said apparatus comprising - a high-voltage transformer, - a reference voltage transformer, - a high-current circuit, - a high-current transformer, - a reference current transformer, - a reference evaluation device, - reference measuring cables (37.1, 37.2), - a container; wherein - the power measurement system comprises a voltage transformer, a current transformer, and an evaluation device; - the high-voltage transformer supplies the required calibrating voltage; - the reference voltage transformer is electrically connected to the high-voltage transformer and is in electrical connection both to the high-current transformer and to the reference current transformer by means of the high-current circuit; - the reference evaluation device is electrically connected both to the reference voltage transformer and to the reference current transformer, in each case via the reference measuring cables (37.1, 37.2); - the high-voltage transformer, the reference voltage transformer, the high-current circuit, the high-current transformer, the reference current transformer, the reference evaluation device, and the reference measuring cables (37.1, 37.2) are disposed within the container; - the sub-components of the power measurement system that are to be calibrated, particularly the voltage transformer, the current transformer, and the evaluation device are in an electrically operative connection to the high-current circuit in such a manner that an on-site calibration on the power measurement system is performable therewith by way of system calibration; characterised in that the voltage transformer is looped into the high-current circuit. - a high-voltage transformer, - a reference voltage transformer, - a high-current circuit, - a high-current transformer, - a reference current transformer, - a reference evaluation device, - reference measuring cables (37.1, 37.2), - a container; - the power measurement system comprises a voltage transformer, a current transformer, and an evaluation device; - the high-voltage transformer supplies the required calibrating voltage; - the reference voltage transformer is electrically connected to the high-voltage transformer and is in electrical connection both to the high-current transformer and to the reference current transformer by means of the high-current circuit; - the reference evaluation device is electrically connected both to the reference voltage transformer and to the reference current transformer, in each case via the reference measuring cables (37.1, 37.2); - the high-voltage transformer, the reference voltage transformer, the high-current circuit, the high-current transformer, the reference current transformer, the reference evaluation device, and the reference measuring cables (37.1, 37.2) are disposed within the container; - the sub-components of the power measurement system that are to be calibrated, particularly the voltage transformer, the current transformer, and the evaluation device are in an electrically operative connection to the high-current circuit in such a manner that an on-site calibration on the power measurement system is performable therewith by way of system calibration;