This depends on the load and on the veracity of the data given by Tesla. A load of styrofoam - which is used in construction, just look what goes under the on- and off-ramps of bridges - doesn't weigh all that much after all. Also, this was not an attempt at with 100% accuracy calculating the range effects of PV panels on the roof - notice the liberal use of the '~' sign and the fact that nearly everything is approximated and rounded down. Did you not notice?