But why involve your car? The logical thing to do is install a large lead-acid battery pack for the house, one which is dirt-cheap and won't suffer being charged/discharged. Lead acid batteries of a capacity equivalent to a Tesla can be had for a few hundred dollars. Lead-acids are also infinitely recyclable and don't involve the nasty/complex chemicals of lithiums. The ideal system would be to slowly charge the massive house battery whenever power is cheap, then dump it into charging the Tesla without drawing from the grid.