Bath County is not the World's largest pumped hydro storage plant. At 24GWh it is smaller than TVA Raccoon Mountain, which is 34GWh. There may be bigger ones, but unfortunately nobody makes rankings of the energy stored in pumped hydro plants.
Re-read the description of Bath County and you will find that they pump from the upper reservoir into the lower reservoir. Therefore, adding the capacity of the two reservoirs and dividing by the flow rate is a bad assumption.
Re-read the description of Bath County and you will find that they pump from the upper reservoir into the lower reservoir. Therefore, adding the capacity of the two reservoirs and dividing by the flow rate is a bad assumption.