There is no such projection, it seems you misread something. The O’Callaghan et al. paper (in the Blog post here the related figure is fig 1) says that current

There is no such projection, it seems you misread something. The O’Callaghan et al. paper (in the Blog post here the related figure is fig 1) says that current (2023) demand is 1500TWh and current supply is 2885 TWh. There are different projections for total demand in 2050 (note all of them project a reduction of demand, due to efficiency gains), one of these is the national grid FES which projects 900 TWh. Importantly the O'Callaghan paper opts to be conservative and choose to use the current demand as the demand for 2050. This is conservative, because it is higher than all projections which all assume that we get demand reduction from efficiency gains.