I am unsure if this qualifies as an answer,
My situation is different to yours as in I have instant gas hot water.
If I were to have a Solar Hot Water System ( Not a Gas Boosted one )
Then all I would do is hit the boost button while my array was producing power during the day and use any excess energy from that. Solar System size would factor in heavily if my Solar Hot water System's element was of a big capacity.
So my Hot water element capacity would also play a role in how much power I could pull from the array and how much from the grid.
I know it is not the answer you seek but i am hoping to provide you with some insight.