Regarding if solar+batteries give cheaper energy than the grid, it's vastly depends! Where and when in the world you are. It's not always that case. You are assuming too much things, sorry. Regarding virtual batteries: that's why I said: "all while maintaining independence of the possible grid surpluses subsidies." 🤷

"Sending power to the grid and then consume it later" is sometimes tricky. Depends on the contract - devil could be in "small letter's paragraphs". Many times you sell power for low price (it's a sunshine, spot price is low) and buy it expensive afterwards (it's a winter). Just small note, that the cost savings are not always as expected ones.