World's Largest Batteries - (Pumped Storage)

I'd say it's a quite big one, "every city" has water towers to provide pressure to the people living in it, or they rely on the water source if it's higher up than the city. There are probably exceptions, where this isn't true, but raised tanks make modern society possible. link more on water towers, which are raised tanks of water. We also pump water into fake lakes to later extract electricity from it. ( ▶ this video ) Another good thing with water towers is that we can keep running the pumps at a lower RPM where efficiency than if we were trying to build a system with pumps to keep pressure. Great channel if you're curious about everything related to humans relationship with water.

Looks like the high-voltage transmission lines are very efficient, losing only a few percent over hundreds of miles: link There's a good watch on pumped hydro, and how it is kinda sucky: ▶ this video Battery storage is the most likely thing, although there is also kinetic storage (flywheels) and some other ideas as well (ultracapacitors, for instance).

You could start by just calculating the potential energy of the volume of the tank you are considering, and the difference in elevation between the reservoirs (volume x 1 kg/L x gravity x elevation difference, plug in the units of your choice and let google figure it out). I suspect the amount of energy is far less than you are imagining... It takes a lot of water to generate a useful amount of energy. This video ( ▶ this video ) does an interesting demo and points out that a 5 gallon bucket on a ladder has the same potential energy as a watch battery.

As I understand it, the main challenge with pumped storage is that you can't do it everywhere. You need fairly specific geography for it, and it's a big civil engineering project. Here's a good video on by Practical Engineering: ▶ this video

Because there are fewer places than expected where it really makes sense. Was discussed in an old video of Practical Engineering: ▶ this video