A 40' shipping container-sized box filled with lead would be roughly 90 metric tons, so multiply that number by three. Now you've got ~45 seconds worth of ener

A 40' shipping container-sized box filled with lead would be roughly 90 metric tons, so multiply that number by three. Now you've got ~45 seconds worth of energy. (An actual shipping container would break before you hit this point; a standard 40' shipping container has a maximum gross weight of ~30 metric tons-- you're not likely to fill it to its volumetric capacity with rocks or lead.)