Solar Project payback's mysterious math

According to the recent article about the 2,419 MW solar electric power array, approximately $250,000 will be saved in energy costs annually, paying back the investment of $5,733,030 in 12.82 years.
Let's see: 12.82 X 250,000 = only $3,205,000.
According to my calculator, payback is 22.932 years.
The ROI is 4.36 percent per year.
What about maintenance?
Will the system last 12.82 years without major repair, let alone 23?
I have an 8.6 kW system myself. Payback is about seven years. So far, so good, because I bought the extended 10-year warrantee. Already, a panel and the entire inverter had to be replaced.
That 10th year is coming pretty soon, however. I hope the WVSD is getting a good warrantee.
Two thirds of the cost was paid for by NYSERDA and income tax credits (read NY and federal taxpayers). I am philosophically against socialistic schemes based on benefitting some from OPM (Other People's Money), I'll vote against them every time.
However, when the voting goes the other way, it's foolish not to take advantage of the benefits.
That may seem hypocritical, but while I take advantage of OPM to benefit me, I am paying taxes to generate OPM for government to give others (after taking a cut for itself, of course).
Likewise, it makes sense for the School District to take advantage of OPM.
However, while making decisions like this to spend OPM and some of our own, let's be honest and clear about the math.
Otherwise, the money might be better invested in T-Bills or municipal bonds issued by other communities for projects they are willing to pay interest for.
John R. Mickowski
BSME, MSEE