I had the same thought. The article does mention pumped hydro storage though. And for that, watts (the capacity of the turbines/generators) is a relevant figu

I had the same thought. The article does mention pumped hydro storage though. And for that, watts (the capacity of the turbines/generators) is a relevant figure of merit just as watt-hours (the capacity of the reservoir) is. The article does use watts kind of indiscriminately, though.