Bob,
I have a 3.9kW array installed & 4.2 kWh batteries on order now for nearly a year. They will we hope (4th time a date has been booked) be installed in early Novemeber 13 months after order. I reckon at 52p kWh the payback is 4.7 years. At the current 29p it is jusr over 8 years.
We generate an average of 11.7 kW per day over the last 4 years. We divert any excess now to heat our hot water first. From May to October we use little / no gas. this year it is 16 units so far (only gas rings on the cooker). We use in the same period 4kWh per day of electricity, so with batteries we would be off grid.
The batteries obviously deteriorate over time so I guess you need to consider if it is replace or supplement the stack. My installation can have 5 batteries on a stack and the 4.2 kWh is only 2 slots. When I last looked they were c£1k each battery.
