Solar savings calculator

I need a simple, light-weight calculator I can embed on my WordPress site to show visitors how much money solar could save them. The tool must accept three user inputs—current electricity bill, average monthly usage in kWh, and their location or ZIP code. Using those figures, the calculator should instantly return two results: their estimated monthly savings and their projected total savings over the system’s lifetime. That is all the end user should see; I do not want any drop-downs or extra steps for choosing different system sizes.