Off grid solar and earth return

[Algarve Sparky]

So despite living working in a country blessed with 300 days of sunshine I have not got my head around this solar thing, there is very few companies here who know what they are doing and buy off the shelf kits and rarely do they perform as sold and in many cases unregistered ones actually cause increased bills instead of feeding in correctly.

Anyhow I have just completed a full rewire on a villa in the back of now where and is totally off grid, they had another company fit an impressive system that we have intergrated into our rewire and it all works well but, for the life of me I cannot see how to provide a working earth return path on the consumer side.

I wired an off grid house previously with old lead acid battery system ( yes I know ) ( fitted by someone else ) and the instruction said the earth of the battery pack was the zero point for the consumer side earth return after the inverter of course, and it worked and was able to achieve good readings etc.

However this new job has a rack style lithium set up which looks great but was very expensive and cannot see how to get any kind of earth return to the consumer side, I fitted a large earth rod to achieve lightening protection for the solar panel grids, but the system itself been off grid has no relation to network ground of course, any ideas guys?

Thanks in advance