You have to do the math, Say the system is 15k and you wrap it up in a mortgage say 30y@5%. At 5% that 15k doubles in 20y to $30k, 37.5k at 30y. Ok so you get this 30% tax incentive. So say that's 5k you don't have to pay taxes on. If your in the 20% bracket that saves you oh, about a thousand. If you stay in the house for 30y when your done the system cost 37.5k Now if you save half your electric bill @$80m it will only take you 468.75 months to break even and at 360m(30y) the system needs replaced.