This is not simple. The batteries in cars can be used to help load-balance but they cannot offer grid-scale storage.
The following is back-of-napkin figures but there are close enough to make the point.
Take a country the size of the UK. It currently uses 1600 TWh a year. Once fully-electric it should need around 1100 TWh a year, so 3 TWh a day.
The UK, despite being a windy place, still has weeks without wind once or twice a year. We had a week without wind power in the middle of August this year and had to generate 60% of our power using gas. A windless week with a frost snap is something you often see in January and February. A cold week, with everyone using electric heating (when solar power does basically nothing) is why the UK might need TWh of stored energy to be secure without fossil fuels.
7 days at 3 TWh is 20 TWh.
A big home Tesla Powerwall can store 14kWh and if run at full power it will be empty in 4 hrs. You'd need over a billion to store 20TWh.
In the UK an average home currently uses around 10kW of electricity a day, but also has a car, a cooker and central heating, all using fossil fuels. Add those in and were looking at 40-50kWh: 20kWh for the heating, 10kWh for the cooking and another 10kWh for the cars.
There are savings to be made of course, the electric versions of all of those items are way more efficient. There's insulation; there's heat pumps; there's electric cars.
The average UK car sits idle for 23 hours of the day. An average-ish car is around 100kW. Even if that hour is slow and often stationary, it's not unreasonable to expect it to be another 10kWh. And the average UK home has more than one car.
A mid-range electric car has a 50kWh battery, so has a bit more beef than a Tesla Powerwall battery. If they are being used for 1 hour a day they are probably going to need a top-up of around, say, 10kW. So let's say they have 40 kWh to spare.
20 TWh / 40 kWh = 500 million cars.
There are only 33 million cars in the UK.
You can, of course, quibble with these figures. There's subtleties everywhere here, but the orders of magnitude are plain. We need to be able to store renewable energy—desperately, and in enormous amounts. Car batteries will help—no doubt—but more in load-balancing and smoothing rather than long-term storage. We need something like grid-scale hydrogen storage to solve this.
See here for 1100 TWh figure. This document has loads of other really interesting details too:
https://www.nationalgrideso.com/document/264421/download