You're missing the point. If you need 24 m^3/day of fresh water, you can have a desalination plant producing 1 m^3/hour and a water tank to match supply to dem

You're missing the point. If you need 24 m^3/day of fresh water, you can have a desalination plant producing 1 m^3/hour and a water tank to match supply to demand. But if you only want to run the plant for 3 hours/day when electricity is cheap, you now need a plant which can produce 8 m^3/hour. There's a tradeoff -- spending more building the desalination plant makes it possible to run at a lower duty cycle so you can spend less on the electricity.