You need to do your own ROI analysis on this because the people who are giving you the figures are also trying to sell you something. They are not disinterested observers.
When you do your ROI, consider:
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Rate of panel degredation. The output of the panels is not constant over time. They will not operate at 100% efficiency and then suddenly die in year 20, they will slowly degrade a bit each year. When we were doing this calculation, a consensus seemed to be that an optimistic estimate was 1% loss of function per year on a cumulative basis, with increasing loss of function years 10 or 15+
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Local arrangement for buying back excess power produced. Unless you’re going to spend major $$$ and get battery backups, you’ll still need to be connected to the electric grid. In theory, you will “sell” the grid any excess power you produce and then buy from the electric company any power you need beyond what your panels produce. But… in most areas, the price of the electricity you buy is higher than the price at which you sell the electricity, so it’s not completely a net neutral transaction. Plus, there may be base fees for just being connected to the grid regardless of how much power you buy. This is where the “gotchas” start in many areas. You really need to understand how the buyback works in your area to know how much you’ll still be paying for electricity. You may find that although your panels produce 100% of the amount of electricity you use in a month, you still have a sizeable electric bill because you can’t store it and need to buy it from the electric company at higher prices.
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What is the duration and quality of your sun exposure? We’re in Florida, so pretty sunny and there are still places where the exposure isn’t high quality (trees, obstructions).
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How will you clean the panels? Any dust, bird droppings, leaves or debris on the panels impairs the output. This is a huge issue in coastal Florida where bird droppings mean that unless someone is spraying off the panels regularly, the output is hugely impacted.
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When will you need a new roof? Putting on new roofing requires removing and replacing the panels - expensive. If your roof isn’t new or expected to be good for 15-20 years (the life of the panels), you’ll need to add in the extra expense of removing and replacing the panels when you re-roof to your cost estimate.