Honestly, I wouldn't believe most of what most solar installers tell you.
What you want is:
1) System Size
2) Cost without the Tax Credits
3) Use PVwatts to estimate the amount of solar produced -
https://pvwatts.nrel.gov/
4) Find all the incentives for your area -
https://www.dsireusa.org/
The rest of the analysis you need to do yourself. You need to know how much you pay for electricity. How much will you be re-imbursed for power sent to the grid. All of this depends on your local utility.