First of all, its 144% additional mass. But the calculation can be done very simply. If the regeneration efficiency would be 50%, then it needs to weight twice

First of all, its 144% additional mass. But the calculation can be done very simply. If the regeneration efficiency would be 50%, then it needs to weight twice down what it weights going up to be a zero sum game. The regneration efficiency should be higher than 50% (Tesla has over 60%) and it is much more than twice the mass going down than going up. So there should be a healthy surplus of energy generated by going down.