The calorimetric system of ITER is designed to measure the time-integrated fusion power output of the tokamak by analysing thermal flows and calculating the energy balance. While too slow for real-time control, its high accuracy is essential for cross-calibrating fast neutron sensors. Implementing this system in ITER is challenging due to the machine's complexity, but the reactor core's enclosure within the cryostat offers significant opportunity to increase the system accuracy compared to the existing tokamaks. This paper assesses the accuracy of ITER's calorimetric system by analysing the entire error chain, from raw experimental data to the final reported results. It explores the difficulties of measuring energy flows in such a complex environment, considers uncertainties from secondary gamma emissions, and examines how calorimetry could aid in licensing fusion reactors. For a typical 500 MW pulse, the system achieves a 2-σ accuracy of for fusion energy output, depending on gamma heating modelling assumptions. The accuracy for thermal power output, unaffected by these assumptions, is approximately under similar conditions.