Comment by jerlam

A non-grid tied charger cannot be depended on. You might get 40km worth of charge. You might also get zero if it's cloudy or the sun is behind a building.

You might say, oh this is fine, anything is better than nothing. But someone cheaper than you will think the same thing, and they will leave their car plugged into the charger all day long, because the cost of free surpasses everything. And it means that the charger will never be available.