Yes, I used some online calculators that predicted how much electricity I generated based on the size of the array, the amount of electricity I used, and how much I could make selling the electricity back, as well as rebates, etc. I can't remember the actual link for the calculator, but it was pretty thorough. Of course, there was a lot of guessing that needed to take place, like the rise in electricity prices, etc.
And as far as I remember, the ability to sell electricity back to the utility like PG&E isn't something that is guaranteed in the future. However, I could be wrong about this point.
One of the things we did was spent a year prior to panel installation reading our meter every day to figure out what our daily power usage was. We then replaced some really energy expensive appliances (and old refrigerator, some incandecent lighting, and ended up with an average of about 24kwH per day)
We just bought our house a few months ago, so this is something I'm doing as well. I'm buying all energy-efficient appliances, and going 100% LED lights. I won't know my total energy costs until I fully move in, though, hopefully soon.
I became aware of this because last year I was hit with an electricity bill of $300 in a month because I ran a space heater every morning. After I did more investigation I realized I was paying about $40/month in electricity keeping my dual-CPU 8-core Xeon server running 24x7. The box alone ended up pushing me into the highest tier, so I was paying the highest rates for the last 5-6 days of the month, so it forced me to reevaluate my electricity use from the ground up.
And as far as I remember, the ability to sell electricity back to the utility like PG&E isn't something that is guaranteed in the future. However, I could be wrong about this point.