None are intended to be very precise, nor can they be. Actual net yield is very site & weather specific. Most online calculators are better than a WAG, but don't expect any more than 1.5 digit precision out of any simple-minded online tool. Assumptions on future kwh price inflation can make a huge difference, and are bound to be wrong (in either direction.)

Real tools with real weather data and the particulars of the site configuration can get you better than 1.5 digits on annual yield, but yer on yer own when divining the cost of electricity 3, 5, 10 , 20 years hence. The cost of financing and any subsidy can be known up front though, once you have an actual quote in hand for hard goods + installation, or whether you'll be allowed net-metering vs. net demand-metering vs. something else (which is state and utility company dependent.) From there you can try to get a handle on whether it makes financial sense in your electricity market.

Maine has relatively high rates per residential kwh on average compared to the rest of the US (~15cents/kwh avg.) But it varies a LOT by local utility and future shifts are hard to predict, eg, as of a coupla years ago Kennebunk was paying about half the state-average, but that's no longer the case- they're now somewhere in the middle of the pack. What may have a lousy ROI based on todays rates may seem like a no-brainer investment five years from now (but don't count on it.) Whatever your rate is now, don't expect it to drop much, eh? ;-)