> Energy is fungible: one cannot compete on "quality" of energy, just on price
It is fungible, but prices can vary according to limitations on supply. If the big company's capacity is maxed out and demand continues to increase, energy already acquired at lower cost and stored in the car can be sold for profit.
But if that limitation is repetitive to the point of investing in infrastructure to use EVs, then some other large-scale investor will close that arbitrage opportunity.
Basically, the next-day / week energy markets, where EV owners can compete, will be saturated by grid-scale battery operators. Renewables will leave large gaps for seasonal energy needs - for example two weeks of winter with no sun and no wind - but EVs cannot help there. So some spin on-demand non-renewables will need to cover that (i.e., the current main providers, after becoming too expensive to run due to carbon pricing).
It'll always be some marginal utility - the main purpose of the EV will always to be a car. You can use it to store energy purchased at non-peak hours so you can avoid using the grid at those times, something that'll probably raise peak prices, because if you need the energy right then, you really need it.
So, EV owners may use their cars to help reducing their energy costs and supplementing their PVs and fixed batteries (if any), but shouldn't expect a car to pay for itself like that.
I see, renewables will only leave gaps that fit your argument. You don't see any scenario where there could be brownouts during the day, say in the summer when AC usage is high?
And no, using cars for grid-scale storage has not been tried multiple times. The technology has never been available/feasible at a large scale before.
There is an economic case being made above that explains why, you might want to try and follow that and respond on point instead of mindlessly nitpicking.
There exist large scale trials for this idea, you never heard of them because (aside from the fact you are arguing on a subject you know little about) they failed or are barely limping along.
It is fungible, but prices can vary according to limitations on supply. If the big company's capacity is maxed out and demand continues to increase, energy already acquired at lower cost and stored in the car can be sold for profit.