Put your upfront costs at the first year's row (you're buying the system).
Calculate how much the system will save you in the current year (365 days), and put that in another column ("energy income") on the first row.
Each future energy income row gets discounted by ~7% multiplicatively: (1/1.07) * prev_row. This is to account for your ability to earn money from other investments, and to discount the future energy flows because of uncertainty. You can tweak this number up or down if you think the regulatory or technical value of solar energy is riskier than I do, or have different opinions about the stock market.
If the sum of the energy income column is bigger than the sum of the expense column, it's a good idea to buy solar. Shop around for the best deal you can get.