[pjc50]

> How inefficient is mechanical storage?

The density is the problem.

If you raise a 25,000kg mass by 100m, that's 24.5MJ, or roughly the energy contained in one kilogram of coal. Or 6.6kWh (standard meter units) of electricity.

(and that's on the input side, not accounting for losses)

[pjc50]

.. or, if you want a fun comparison, a 2000kg Tesla with a 100kWh (360MJ) battery pack would have to be raised over 18km into the air to equal the energy stored in its battery.

[heavenlyblue]

That’s not how water storage works though for example. You are forgetting that for every unit of mass raised 1 unit, you can add another unit of mass where the previous unit was. Thus you are getting a progression of 1+2+3+4…up to the height units of power stored.

[maxerickson]

The water tower in my town is several hundred feet tall (many units) and has a big bulb tank at the top. When you pump a gallon of water up into the tank, it is raised hundreds of feet and contributes a fraction of an inch to the total height of the water stored.

It's an actual implementation of energy storage, the water is pumped up there and then gravity distributes it, so there is a buffer if the pumps can't run for some reason.