You can normally sell the excess energy back to the utility. But what, and how much you need depends o lot on where you live. According to this: http://www.nytimes.com/2007/04/14/business/14money.html a solar system makes a lot more sense in southern Cali than in New Jersey.

 

I'm guessing that the system size in the article is 5000-6000 Watts based on the price mentioned. My area has incentives just a little lower than New Jersey apparently, so it would be maybe 15-25 years before I would see savings. Ouch!