Base it on the incentive limit, Find out if your state has a limit at www.dsireusa.org . That’s It!

The payback of any solar systems without incentives is probably 35 years. With a 30% federal incentive it brings it down to 25 years. Most of the good state incentives allow you to take around another 35 to 50% off, but many have a limit.  If you go pass that limit, your payback can go to around 15 years instead of 5 years or less.

Stay within the limit. Thats the size of your system.

-John Kimball