If solar were free, but we still needed to pay for battery storage, how would it then compare in cost to fuel-based alternatives (fossil fuel, nuclear etc)?
It's relatively high, the problem is that building new dams is an environmental disaster, and existing dams are two orders of magnitude below needed capacity.
Also, hydro dams kill a lot of people when they have accidents.
Do you have to dam a river to store energy in this way? Can they just build water towers that pull water from underground up into a tank and release it via gravity to generate power when needed?