Current aortic stenosis severity grading is based mainly on the local properties of the stenotic valve, such as pressure gradient or jet velocity. Success rates of valve replacement therapy are still suboptimal, so alternative grading of AS should be investigated. We suggest the efficiency of power transfer from the left ventricle to the aorta, as it takes into account heart, valve and circulatory system. Left ventricular and circulatory power were estimated using a 0D model, which was optimised to patient data: left ventricular and aortic pressure, aortic flow and diastolic left ventricular volume. Optimisation was performed using a data assimilation method. These data were available in rest as well as chemically induced exercise for twelve patients. Using this limited data set, we showed that aortic valve efficiency is highly heterogeneous between patients, but also often dependent on the haemodynamic load. This indicates that power transfer efficiency is a highly interesting metric for further research in aortic stenosis.