If you drill a hole 1500m deep, and you use concrete as the material of the object used for energy storage, and the volume of the object is V [cube meters], whe

If you drill a hole 1500m deep, and you use concrete as the material of the object used for energy storage, and the volume of the object is V [cube meters], when the object is at its maximum level you have stored an energy of: E = 10 x V KWh This means that with 10 cube meters of concrete you can store 100 KWh (more or less the energy consumed by 10 households per day). If what they claim about doubling the energy storage is true, with the same amount of concrete you can store 200 KWh. 10 cube meters of concrete would weight just 24 tons, not so much after all. Assuming a hole 1600m deep, and a system of cables capable of handling 250 tons (100 cube meters of concrete), you can store up to 2000 KWh (so you can serve 200 households for a day). I don't know the price of drilling a hole 1600 meters deep, but with these premises, if the price is in the range of 1-5 millions $, a medium neighborhood can afford the operation.