[en] The use of the Hall C achromatic arc line as an energy analyser is proposed. It has a dispersion of 12cm/% with all the quadrupoles, sextupoles, and beam correctors switched off. The transverse position and the angle of the beam at the entrance of the arc is precisely measured by a pair of wire scanners spaced by 2.5 m, and the transverse position of the outgoing beam is measured by another pair of wire scanners at the exit of the arc. After the absolute beam energy is measured, the arc will be turned into normal achromat mode by energizing all the elements and the beam position probe located at the mid-point of the arc is used to monitor the beam energy in the operational mode. A complete error analysis shows that an absolute beam energy measurement with 10^-3 accuracy can be achieved. Relative energy measurements at the 10^-4 level are also obtainable