[David]

Your home charger won't be 250kW since you have 8 hours or more to recharge. My home is so much more efficient now than when it was built 40 years ago that the extra 7kW for normal charging wouldn't be a problem.

When cars and the grid are smart enough to know when is a good time to charge and when to discharge, they will also be smart enough to look at your calendar and know if you have a 40 mile commute or a 300 mile road trip. You could also tell the car not to discharge more than x amount of power.

We have a large battery plant for load stabilization out in West Texas near the wind farms. It's constantly charging and discharging. When it discharges it's for seconds at a time or less so we're not talking about cars selling large amounts to the grid but rather smoothing the load when a large load comes on until a slower gas plant can catch the load. Unless you want to sell more to the grid perhaps when there's a spike in electric rates and your car would smart enough to do that or any number of other charging and discharging scenarios.