Yeah, I just wanted to go over energy rather than power since it's what is usually more important to the discussion. Another comment mentioned efficiency, so I didn't bother with it.

The 50k kg was definitely a mistake on my part. I tried editing it from 1k to 50k cubic meters in the example, but must have changed the wrong 1,000.

It's probably somewhat common knowledge, but the physics are pretty straightforward.

Energy = (mass of the water) * (acceleration of gravity) * (height it goes down)

Acceleration of gravity is ~10m/s/s

Height is in meters, usually several hundred for these projects

Mass of the water is in kg. 1 cubic meter of water is 1,000kg, so you can get the mass straight from the size of the reservoir.

The energy will be in Joules. For there are 3.6 million joules in a kwh, so divide to get it in kwh.

So, if you have 50k cubic meters of water going down 500m, that would be

(50,000 * 1000)kg * 10m/s/s * 500m / 3,600,000 ~ 70,000 kwh

50,000 cubic meters of water would be 10m x 50m x 100m, for example.