A lot of the solar quotes I've gotten will take your current usage to show a cost-benefit analysis. What I want to see is what happens to that benefit when my usage changes. When sizing our prospective array, I want to see how much more electricity I can use before the costs really blow up. Ideally, I want to keep grid usage in the lowest 3 tiers. So I created this spreadsheet to help with planning.
It's based in Google Docs. It's read-only but if you go to File->Make a Copy, you can save yourself an editable copy. There is a 2nd sheet in the spreadsheet called 'info' that contains instructions on how to get the data for your specific scenarios. The information is based on California's PG&E standard residential service. You can adjust the formulas, tiers, rates, etc. based on your situation. Also note that many solar households go to the time of use rate schedule instead of the standard schedule, so that will affect calculations. For simplicity, I've chosen the standard schedule. Let me know if I've mucked anything up.