Just one Powerwall costs more than the cost of burying an underground gas tank, and you'd need 10 of them to store enough power to charge one Tesla 100 kWh car.

Just one Powerwall costs more than the cost of burying an underground gas tank, and you'd need 10 of them to store enough power to charge one Tesla 100 kWh car. That's $35,000 in batteries alone without the cost of installing them. $35,000 is much more than it would cost to put down an underground concrete foundation to drop a flywheel into. House-size basement foundations can cost 1/8th that.