The devil is in the deterioration of the battery - only operating at 60% after ten years.  It's the same people who put in my solar panels, and they were good to deal with and it is a sale price -$12k including installation.  My little brain goes away when I try and contemplate the maths but my sense is if the cost of electricity stays stable then over the 10 years it's about equal to the cost of my electricity bills.  And as things stand I pay for the power I use and then I get paid for the power I put into the grid and that was okay when there was a government subsidy but not any more.  With a battery I can set up to use my power first and the big benefit is no power cuts as well as the possibility of my electricity bills ending up in the positive, it's very sunny here.  And then, even if the battery is at 60% it's still there and it might be in good enough shape to last a while longer -  so I went ahead, hopefully it can't go too far wrong - they have said it will be installed in 3 weeks and I've given them the deposit.

and thanks edrek too, that confirms what I've read which is always good.  

last firing for the verandah glass yesterday - I am still tired this morning but all cool, not perfect but worth it for the colours and it looks like the builders are getting ready to do it soon.  I am just going to close my eyes and take the quote because I know he's a good builder and it will be reasonable.