low energy feeds (e.g. high in fibre, for rest, light work-moderate and good doers) contain about 7-9 MJ/KG, medium energy feeds (for those in modertae-medium work, those needing a little extra condition or sparkle, some poor doers and veterans) contain about 9-10 MJ/KG and high energy feeds (for those in hard work, some veterans, poor doers, weight gain) contain about 10-12 MJ/KG.
sugarbeet provides 9-10 MJ/KG, so is a medium energy feed but releases it slowly (as it is high in fibre and low in starch). however, though it being low in sugar despite its name, it may be a good idea to go for an unmolassed version, such as speedibeet or badminton. this menas your horse would be less likely to be excitable, yet still maintain good condition throughout the winter.
good luck!