I still have at least 6 weeks before any cells arrive, so feeling bored, I thought I'd add a financial aspect to the power and energy audit, making it a power, energy and financial audit.
I've just rediscovered that the grid-tied Fronius inverter, connected to the panels on my house roof, stores the total energy generated over the life of the system. I can combine that with the data I already have on energy exported, to find the total amount of energy I have consumed from the panels.
So over the 1243 days the system has been in operation. it has generated 38,861 kwh. Of those 23,328 kwh were exported while 15,553 kwh were consumed locally. At local prices prevailing when that power was consumed, the saving was $4,459. Add to that $1,381 taken off bills for export credits, the total saving is $5,840, which is more than I paid for the whole system. I am quite surprised at the shortness of the payback period on the solar only grid-tied system.
Anyway, the figure of interest for the current exercise is the $2,959 spent on imported energy. That figure alone tells you that the batteries, which have a shorter expected life than the panels, will have to work harder to pay for themselves. Making it even harder, some of the imported energy came in during the day when there was cloud cover. This whole thing would be so much easier if the batteries were in the same circuit as the house panels, so they could just cut in and cut out as needed, but the regulations make that very difficult.
Nevertheless, if I contain the costs on an eventual 16 cell system, I should be able achieve crude cash payback within 5-6 years.