Time value of money scales with the value too, though. I just wanted to make the math easy. If you the system costs half, and delivers half the savings annually, the ROI is the same. If the system costs half and saves a little less annually, the ROI is a little longer. I'd like to see real numbers where a 100% system gets you a ROI of 5 years and a smaller system gets you a ROI of 10+ years.

> $17k right now is worth incredibly more to me than saving $200/month for the next 20 years.

Would you take a $17k loan today, and pay it back at $200/month for 20 years? That's a 13% loan. Which I guess is a market rate for a personal loan, but those usually have a shorter period. My preferred credit union gives solar loans at 6.25% to 8%.

Would you take a $17k loan, to save $200/month for 20 years, but have to pay $191/month for 10 years to clear the loan? What about if you paid $146/month for 15 years? If not, why not? How about if you paid $331/month for 5 years?