It matters because the 30% number is an overall average. Some days it might be 5%, other days 40%. The 5% days are generally few but often high overall demand days (mid summer). You can’t save several days of more than average for next week when it will be high demand and low output. Of course, technically, you can with a big enough battery. But it doesn’t make economical sense to do this when it happens just a few times per year. There have been many studies on this over the years.
Mike