As you point out, it depends on the plant. In our case, nameplate rating is 750 MW on each unit, and they can run for extended periods at 200-250 MW, so 30% or so.
I am not adverse to expanding renewable power, but the real problem at this point is still backup power. Wind/solar/hydro are not always available at anything approaching rated full load. Without either viable storage systems or fossil-fuel/nuclear backup, there are going to be times when there is just not enough power in a region, and there is not enough inter-grid power transfer (in the US) to make up the difference. People are not going to like being in the cold and dark for extended periods.