Hi There! There are many sites out there - But basically it needs to gather information to include these steps:
Customer Inputs average Monthly Bill amount
Customer selects energy provider (SDG&E, Edison, or PG&E)
Customer Inputs zipcode/type of project (single family, condo, manufactured home, or business)
Customer receives Calculation of how much solar can save them based on their bill and energy provider.
Each calculator is a little different, some more sophisticated than others - Here are some sites with calculators: https://www.goadtsolar.com/ https://pvwatts.nrel.gov/ https://sunroof.withgoogle.com/ https://understandsolar.org/