Abstract This paper describes a method for the efficiency determination of a cavity receiver using the example of the solar hybrid gas turbine system SOLUGAS. Major focus is given on the improvement of a new approach of the solar flux density determination based on a measurement-supported simulation technique where an acceptable uncertainty of the solar input power of −1.3%…+6.3% is achieved. For the thermal evaluation an uncertainty of 2.4% is determined that leads to an overall uncertainty of the thermal receiver efficiency of −2.8%…+7.7%. Detailed uncertainty propagation is presented and conclusions discussed.