> On another note, calculating the payback of 
   > residential solar panels is tricky because many
   > online calculators don't consider (stuff...)

Sometimes they do, we installed Solar in 2003, we replaced a failed inverter in 2016. At the time we installed the system we got a new roof with asphalt shingles (50 year roof) which both helped the roof load (they were lighter than the previous wood shake) and was a time period longer than the payback period for our panels.