I did my NPV calculation with probably the wrong interest rate, but I'm also looking at the 10 year NPV of 80% of our kWh charges as my guide (including the published rate increases).

If it maths with that, great. If not, but it's close, we'll look again when we need to do the roof. Utility prices are rising, panel prices are dropping, it'll probably make sense eventually... Installer costs are going up too though.