Outline the perimeter of the plant with a closed way. Add power=plant and add all additional relevant tags. Add landuse=industrial to the perimeter unless the plant is underground or limited to a single building. Buildings that are part of the plant are normally tagged building=industrial.
Many hydro plants in Norway are located inside mountains. Make sure you add location=underground to these.
Adding the total output of a hydro plant to the each individual turbine.
Each turbine will have its own value that is added to get the total output, so unless there is only one turbine this will be wrong. Finding each turbines specification can require a lot of research as it might not be mentioned on the relevant websites.
Assuming that all turbines in the plant have the same power output.
They may have been built at different times or been upgraded and can have different specifications.