(a) Monthly payments. The loan shall provide for monthly payments on the first day of each month on account of interest and principal and shall provide for payment in accordance with the amortization plan as agreed upon by the borrower, the lender and the Commissioner.

(b) Amortization period.

(1) The loan shall have an amortization of either 5, 10, or 15 years by providing for either 60, 120, or 180 monthly amortization payments. No energy saving loan shall have an amortization period in excess of 15 years unless the amount of the loan exceeds $50,000.00, in which event the amortization period may be increased to 20 years, with a provision for 240 monthly amortization payments.

(2) In any event, the loan shall have a maturity satisfactory to the Commissioner of not less than 2 or more than 20 years from the date of the beginning of amortization, or the Commissioner's estimate of the remaining economic life of the structure, whichever is the lesser.

(3) The Commissioner shall establish the date of the first payment to principal, which shall be no later than the first day of the second month following the date of final endorsement (for projects involving insurance of advances) or endorsement (for projects involving insurance upon completion) of the loan for insurance.