When electric cars become more popular, the demand for electric cars will increase, and quantity of electricity consumed will increase. Therefore, the production of electric cars will also increase in equilibrium. Whether increased production will increase or decrease average total cost depends on the relative importance of two forces: (1) on the one hand, increased production leads to lower average fixed cost; (2) on the other hand, increased production leads to higher marginal cost, due to diminishing marginal returns. Hence, if the first force is more dominant, then average total cost will fall. If the second force is more dominant, then average total cost will rise.