I've just had a modest 4.3kW system with a battery installed. Pay back time is about eight years which is probably beyond my tenure but so far I'm pleased. Cost was £9k including car charger. I live alone of course and go on holidays a lot so there will be plenty of times when the battery is full and it's exporting to the grid.
The overnight cheap tariffs are not an option with OVO so I shall need to explore alternative suppliers once I have more analysis of the system.
I recently bought a PHEV too so I seem to be turning a bit green.