> they normally pay for themselves after something like 10 years from memory

I would guess this varies by locale, and people usually assume the time value of money to be zero when they make the claim. In much of the US, I think the payback time is more realistically in the 15-20 year range, which means you really only want to install panels when you install a new roof. Otherwise the lifetime of the roof will happen before break even on the panels.