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The whole idea is interesting, and signals how the electricity grid might change in the future, especially as electric vehicles—made by Tesla and others—proliferate on U.S. roads. The Ford Motor (F) F-150 Lightning, for instance, can power a home for three to 10 days—depending on what gets hooked up—in the event of an outage.
The Tesla/PG&E program is only available to customers with solar panels. Otherwise PG&E would just be paying for electricity that the battery took in from PG&E’s existing generation sources. Right now there is no way for Tesla vehicles to deliver power back to the grid, the way an F-150 can. And most EVs are charged at home without solar panels.
Still, all the batteries in EVs already on U.S. roads amount to the equivalent of roughly 11 million Powerwalls. They could be another interesting asset utilities might want to tap some day.