In high vacuum flat solar thermal panels, the main losses are the optical and radiative losses of the selective solar absorber. The optical properties of commercial selective solar absorbers are usually measured by reflectivity measurements from 350nm up to 20μm at room temperature. The spectrally averaged absorptivity and emissivity are estimated and their values are used to calculate the panel performance at high temperature. To measure the actual absorptance and emittance at operating temperature and up to stagnation temperature (as high as 400°C), we have developed a calorimetric instrument based on a power balance equation. We compare the results obtained by our instrument based on a calorimetric method with the value from the optical data. The results are in excellent agreement in the infrared region whereas the calorimetric method measures a spectrally averaged absorptivity 0.85, lower than the spectrally averaged absorptivity measured by optical method by about the 10%. An optical energy loss mechanism could be responsible for the observed discrepancy: further investigations are in progress to clarify the origin of the discrepancy.