Feed to the grid.

I have a 21 kw solar system with a SHP2 and DPU. If my batteries are full, will the system charge anything on the SHP? Also, if I’m overproducing above what the panel is drawing, will it send excess to the grid? Customer support says no, but I can’t seem to wrap my head around this and as to why you cannot feed to the grid to earn credits, according to them.