Similarly to electron beams in betatrons, high energy runaway electron (RE) beams require significant vertical field to be kept at a stable radial position within the tokamak chamber and this field is clearly a function of the average electron energy. This effect appears to be very significant in small tokamaks like COMPASS (Ficker et al., NF 2019),(Vlainic et al., Atoms 2019), allowing for rough estimate of total beam energy or average energy per particle. However, JET magnetic equilibrium data also suggest that this effect may be observed in runaway electron experiments at large devices as $\beta_{N}$ values during the beam plateau phase are significant despite the negligible temperature of the background plasma. In this contribution we try to compare the information about the RE energy evolution estimated from magnetic equilibrium with intensity of synchrotron radiation. The patterns of the synchrotron radiation are modelled using SOFT (Hoppe et al., NF 2018) with energy estimated by the aforementioned method as an input parameter and also compared to the patterns observed by mid-IR ($3.25 \pm 0.25\,\,\mu m$) and visible range cameras during the experiment.