You can but that doesn't help if there is no surplus water to store. You can store 82TWh and your yearly consumption is about 500TWh. NO3 and NO5 are pretty much at their lowest points in the last 28 years, it's not a problem at all though because right now that's about 71% and 61% respectively. There are of course up- and downsides to being connected. An important upside would be that when you're connected to other markets you can import electricity when the price is low and pump the water back into the reservoirs to release it when the price is high again.