The energy of extensive air showers can be determined with the energy radiated in the form of radio signals. The so called radiation energy can be predicted with modern simulation codes using first-principle calculations without the need of free parameters. Here, we verify the consistency of the radiation energy calculations by comparing a large set of Monte Carlo simulations made with the two codes CoREAS and ZHAireS. For the frequency band of 30 – 80 MHz, typically used by many current radio detectors, we observe a difference in the radiation energy prediction of 4%. This corresponds to a modelling uncertainty of 2 % for the determination of the absolute cosmic-ray energy scale. Hence, radio detection offers the opportunity for a precise, accurate and independent measurement of the absolute energy of cosmic rays.