Just for argument sake
20 panels with 2 Amp @ short circuit. So we will get probably 1.5 amps at 12 volts.
So 18 Watt panels times 20 is 360 Watts
5 hour days of ideal sunlight is 360 X 5 equals 1.8 KWH per day
Power produced in 365 days (if you are very lucky and have no clouds) 657 KWH
So the power company pays say .06 cents per KWH would produce $39.42 per year.
I can buy power at .06 cents per KWH now so it will depend on what your power company sells it for but generally 1/2 of that.
Forget the grid tie invertor and pick up one from Wally mart and a couple of batteries.
I have 18 123 watt panels, A Bergy XL 1.2, and a listeroid 12 hp running on free fuel and can't justify trying to sell it back to the grid. At least check very carefully with your utility before running out to spend money.
Just my opinion for what it is worth. My wife says nothing.
Ken