Yearly savings are the share of the electric bill you say the system covers. Payback years are the system cost, minus the incentive amount you type, divided by that yearly savings. Incentives are only the dollar amount you enter. The page does not decide that a tax credit applies.
A $20,000 system with a $0 incentive and $1,000 a year of bill savings pays back in 20 years. Type a $6,000 incentive and the net cost is $14,000, so the same savings pay back in 14 years. The incentive has to be a number you believe applies.
It is not a US federal tax-credit determination, and it does not know a utility’s net-metering rate. Panel degradation, a loan on the system, and rising electric rates are not projected unless you have already folded them into the savings figure.