German Wikipedia suggests the power of each generator is 16.400 HP, or ~12MW, bringing the total power to ~120MW. (cited source: Schweizerische Bauzeitung, Heft 24, Band 63/64 (1914)) That is not quite exactly the 108MW so there might be some confusion going on regarding watts and volt-amperes (or, in other word, effective vs. apparent power).
The local tourism website offers 14.500 HP to make everything more fun: https://www.visitrjukan.com/de/theme/rjukan-and-notodden-on-unesco-s-world-heritage-list/vemork-power-plant
Reading between the lines in that link, it is also possible that the turbine/generator sets were running slightly below max power in order to increase efficiency (i. e. electric power output per liter of water throughput).
So it seems to me that the 108MW figure is accurate at least for the nameplate / nominal power capacity, with actual capacity possibly differing by a couple of percent, as it does with any large power station.