I think using the car batteries as storage also makes a lot of sense. Suppose technology improves so that cars have ~double the capacity they have now (~150kwh), then that’s super useful for long car journeys, but most people only do short journeys each day. So you could use the excess for storage. And they’d be plenty considering the average household in the uk only uses 10kwh a day (although admittedly that’s factoring in gas heating which will need to be replaced)