Select your state on the calculator page (found in the Resources section). Once you've selected your state, indicate your specific region within the state based on the selections offered.
Adjust the "PV Systems Specifications" fields if you know them to be different from the default settings. If you do not know these specs, leave the default numbers in place to receive a close estimate based on your city/region selection.
Enter your current energy/electricity costs in the form of cents per kilowatt hours. If you are unsure of this number, leave the default in place.
Press calculate to generate a monthly and annual solar energy production estimate.
Multiply the PV STC value by .8 (80 percent of STC). The "STC" is the watt rating of the solar panel or system. For example, if your solar panel is rated at 135 watts, you'd need to find 80 percent of that, which is 108 watts. Because the 135 watts is a best-case scenario, 80 percent of that rating gives you a more realistic projection.
Multiply your wattage number by the number of hours of usable light. Five to six hours per day is standard. Note that even though the sun may be up for 10 to 12 hours, the first and last three hours of light are weak and thus will not greatly increase your solar production.
In our example, 108 x 5 = 540 watts per day.
Subtract the number of cloudy days away from 365 and multiply your daily watt output by that number. In a perfect world you'd have 365 days of pure sun, in which we'd generate 197,100 watts per year based on our example numbers. Again, we want to find a more realistic average. You now have a rough estimate of the solar production you can expect per year.