Sure, looking at my spreadsheet I also discounted the cost by the same interest rate after 30 years, (1/1.04)^30. Agreed that including the second system doesn

Sure, looking at my spreadsheet I also discounted the cost by the same interest rate after 30 years, (1/1.04)^30. Agreed that including the second system doesn't really make much sense, but I wanted a quick-and-dirty estimate without having to individually discount each year's production (as you would if you had a finite time horizon).