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Originally Posted by P-hack
Separate concerns, and applications.
1. A house doesn't need the same power density battery as a car.
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True. However, if a vehicle already has enough energy to supply the house in an emergency, then the extra expense of a separate house battery is unnecessary. While some people may find it worthwhile to have a separate house battery, those with more modest budgets would be best served to have their car battery pull double-duty.
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2. A car needs to be charged to be useful, can't drain it overnight and expect to drive it the next day.
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Intelligence must be programmed into the system so that the vehicle owner can specify the depth of discharge before the vehicle stops supplying energy to the house/grid. The utility should provide an economic incentive for the customer to volunteer a portion of the vehicles electricity, but allow the customer to determine what portion is needed for their driving needs.
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3. The house should still function when the car is not there (and have batteries available for storing daylight energy).
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Necessary for an off-grid home, but the vast majority of customers are on-grid. Schedule the vehicle to charge off-peak, but volunteer the vehicle energy to cover peak demands.
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If anything the house needs a much larger battery, so it can charge the car overnight when it is home from energy stored during the day and supply the household needs overnight.
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Peak leveling is more economically accomplished on a large scale. For off-grid, a battery is a necessity. For the majority of customers at the moment, a large-scale peak leveling mechanism is cheaper. Plugging in an EV to the grid is just an extra benefit to leverage a resource that already exists.