If you have one 6kw inverter for PV and battery, then a 8.5kw shower will take at least 2.5 kw from the grid for the time it is heating water fully. Are you seeing much more than that? Do you have a monitoring app that will show details?

I would say it sounds fairly normal @Jackol . We have a similar set up and depending on the time of day, generation level and other draws on the system, we see quite a draw from the grid.
Also depending on your inverter, they can often kick in some time after the switching of a significant power source and so you would see the grid input instantaneously making up the difference. 

Thanks very much for that. That's what I thought but reassuring to know others have the same opinion.

Thanks very much for your experience. It looks like everything is okay. I just wanted to check it was what I should be expecting before I paid the bill!!