If the item is a major improvement such as a new furnace or something like a solar panel then I might go with 20 years.Then you simply run the CAGR calculation using a future value of the annual savings * lifetime of the item.That gives us an equation of:expected % return = (( annual savings * lifetime in years / cost ) ^ ( 1/lifetime in years) ) -1If you are not certain on the values then you sho