So 1 ton for 200 kWh, so 200 Wh/kg.
1 ton of concrete drops from 4000 m. Using formula: 1000 (masse) x 9.81 (gravity) x 4000 (height) = 39,240,000 Joules = 10.9 kWh.
Now, in water, taking account for Archimedes’ buoyancy. Concrete density: 2.4 tons/m³. 1 ton of concrete is 0.416 m³. The downward hydrostatic force is 1000 - 416 = 584 kgf = 5729 N. The gravity becomes 5729 / 1000 = 5.729. I use again the formula: 1000 (masse) x 5.729 (gravity) x 4000 (height) = 22,916,000 Joules = 6.365 kWh.
Air or water resistance is not taken into account. In practice, even water resistance would be low given the low descent speed, which is stabilized at about 2 m/s.
Now the first video indicates a speed of 8 kmh, so 2.22 m/s, leading (for 1 ton and by kinetic energy) to 1000 x (2.22)² / 2 = 2469 J per second, so 2469 W until stopping. The time with 4000 m height is only 4000/2.22 = 1800 seconds. So 1 ton of concrete falling from 4000 m at stabilized speed of 2.22 m/s in water will produce about 1.2345 kWh, and the double (2.469 kWh) if falling from 8000 m at stabilized speed of 2.22 m/s (calculations to be verified).
The claim of 2x less density for concrete compared to Tesla battery is largely exaggerated, and the number I provide is about 160x less, 200 kWh becoming 1.2345 kWh. But the gravity energy storage in water can be interesting because concrete is cheap (200 $ per ton) and has a long lifespan, unlike chimical batteries.
I propose using sandbags. Sand is still far cheaper (and non-polluting): 25 $ per ton. If it is 200x less efficient than Tesla battery, it would be equivalent to a Tesla battery of 5 kg, so 1 kWh. And a Tesla battery of 1 kWh would be far much more expensive than a ton of sand.
Some calculations with sandbags I propose:
For a sandbag of 320 tons (320,000 kg), diametrical section = 10 m², height = 20 m, volume = 200 m³, sand density = 1,600 kg/m³, the downward hydrostatic force is 120,000 kgf, so 1,177,200 N, the limit velocity is about 21.7 m/s with a drag coefficient of 0.5, and the “gravitational acceleration” (force in N / mass in kg) is only (because of the aquatic environment leading to Archimedes’ buoyancy) 3.67875 m/s² instead of the usual 9.81 m/s². Note: 320 000 (kg) x 3.67875 = 1177200 (N). Falling at 2 m/s, it generates 320 000 x 2²/2 = 0.64 MW until it stops.