Recently, the energy determination of extensive air showers using radio emission has been shown to be both precise and accurate. In particular, radio detection offers the opportunity for an independent measurement of the absolute energy of cosmic rays, since the radiation energy (the energy radiated in the form of radio signals) can be predicted using first‐principle calculations involving no free parameters, and the measurement of radio waves is not subject to any significant absorption or scattering in the atmosphere. Here, we verify the implementation of radiation‐energy calculations from microscopic simulation codes by comparing Monte Carlo simulations made with the two codes CoREAS and ZHAireS. To isolate potential differences in the radio‐emission calculation from differences in the air‐shower simulation, the simulations are performed with equivalent settings, especially the same model for the hadronic interactions and the description of the atmosphere. Comparing a large set of simulations with different primary energies and shower directions, the systematic uncertainty of the radiation energy is obtained from the observed differences, amounting to a total of only ~3%. ... mehrThis corresponds to an uncertainty of only ~1.5% in the determination of the absolute energy scale and thus opens the potential of using the Radiation energy as an accurate calibration method for cosmic ray experiments.