The idea is not to fully discharge it overnight, but to smooth out demand. During the daytime, solar power is plentiful, but not during the evening. However, d

The idea is not to fully discharge it overnight, but to smooth out demand. During the daytime, solar power is plentiful, but not during the evening. However, during the evening, there is demand from residential customers so that they can cook, watch TV, use their computers, and whatever else people do in the evening that uses electricity. Because there needs to be as much energy supply as there is demand, peaker plants (coal or gas typically) run during the evening to compensate for the fact that solar generation isn't available in the evening. There is less demand for power overnight though, since most people are sleeping. As such, what would make sense would be that people would recharge their cars during daytime, drive back home, connect their cars to the grid and have them contribute some percentage of their battery to lower the need for peaker plants in the evening, then charge to full overnight.