I recently replied to another about solar in a different thread. Here's part of the message that's relevant, plus more.
I have had 6.35 MW panels in MA for almost 3 years. (I think twenty five 255 W each) I love it. Probably 7-8 year break even.
Fed tax CREDIT is 30%. I think it expires in 2016. I also got a MA rebate of 4k and a few other small perks, like the system cannot be factored into the value of my home for property tax purposes for 10 years in thee state of MA.
Ideal system is south facing, little or no shading, and roof slope degrees equal to latitude degrees, but you can definitely do well if you pnly have 1-2 of those. Get a system that meets a bit more than your yearly needs in case you have unexpected electricity demand.
Remember the panel watt rating is lab tested at 25 C under nearly perfect conditions. You will probably not get that rating except on cold bright winter days. Panel effeciency increases by 1% every 1 degree less or so. Thus I tend to max out in Jan. or Feb. but only for a short time because of course day length is much less. Plus I get a bit more shading from some trees.
Panels also degrade 0.5-1%/year due to UV light exposure. Essentially they start to "fog up" slightly until they are at ~85% of what they were rated at. At that point they reach a saturation point and will not fog up any more, so just realize this.
I have National Grid in MA. I pay $4/month for the privelage of hookup to the grid to sell back my excess power at the same rate I would buy it. Excess accumulates as credits. I got ahead for a year, then bought a chevy volt plug in hybrid electric. I figured I would do 70% electric miles, but to date have done 92% all electric. I tried to figure this in when sizing my PV system.
SRECs (Solar Renewable Energy Credits) are currently selling in MA for high two hundreds per MW. I made 6 MW my first year and 7 my second. Unused Fractions are just carried over to the next quarter.
Also, lots of people don't realize that if you are grid tied, when the grid goes down, you lose power too. there are some battery grid tied hybrid systems out there, but batteries are really expensive and need babying with discharge and such to preserve life. Perhaps you will start to see lithium based batteries and that will make a positive difference.
Good luck.