A 40&#x27; shipping container-sized box filled with lead would be roughly 90 metric tons, so multiply that number by three.  Now you&#x27;ve got ~45 seconds worth of energy.<p>(An actual shipping container would break before you hit this point; a standard 40&#x27; shipping container has a maximum gross weight of ~30 metric tons--  you&#x27;re not likely to fill it to its volumetric capacity with rocks or lead.)