The panels can supply all your electric if you install enough panels, lets say 33. For Duke you generate power in the day and send the excess to Duke and they nicely return it to you at night at no charge. At the end of the year (Dec.) if you have excess power they pay you at I think the whole sale rate (very low rate). You start all over in Jan so you could have a small bill that month, but again it depends on the number of panels and how much sun you get for the month. Duke also charges you about $10 a month but this can be paid from the electric you use. (Note when Duke's power goes out your panels stop producing for safety reasons.)
Payback is long and if you are doing this just to make money you probably wan to forget it.
One way to look at it is what you can earn on the money you would spend on solar if you put the money in the bank and used the interest to pay your electric bill. Remember to add in taxes on the interest income. You will have to estimate rising electric rates. Lets say after tax rebates the system cost you 20,000 (probably a little higher) and supplies 100% of your electric. Lets say you spend 1200 a year now on electric. That means your 20,000 has to earn about 6% plus maybe 0.25% to cover taxes. You won't do that in a save investment. Thus you could look at investing 20,000 in solar and you never get it back (but you had no electric bills) vs say putting 60,000 in a 2% bond and leaving the bond in your estate. Its your choice. By the way if you assume a 20 year life for the panels and the 1200 per year does not increase you saved 24,000 in electric bills. This is just a way you might look at return and naturally you have to put real life numbers into your analysis. I would also add an estimate for removal and replacement when you put a new roof on and you may have some maintenance expense.
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