Quinny:

 

I see people quoting a 30 year ROI and it makes me laugh. I spent $36k and don't regret it one bit. My property value was increased by QV close to this. 

 

[snip]

 

I will have paid off around half of the money in 5 years based on power savings and utilising Westpac's 10k at zero %.

 

Everyone's property value has increased by this much. Or more.

So you're looking at a 10 year ROI, at which point you'll be replacing the batteries? But presumably with better batteries.