Section 60. A credit union may make loans to the owner of improved real estate for the purpose of financing the improvement, repair, alteration, rehabilitation thereof, or fuel for supplying the energy needs of such real estate or the purchase and installation of fixtures to be affixed thereto including, but not limited to, the purchase and installation of a solar or wind-powered system or heat pump system. Such loan shall not exceed forty thousand dollars exclusive of interest or discount from the date of the note, with respect to any one parcel of such real estate. The loan shall be payable within fifteen years from the date of the note. The terms of the note or mortgage shall require that, commencing not later than two months after the date of the note (a) payments shall be made on account of the principal in equal monthly installments in amounts which, at the maturity of the note, shall equal the original amount of the loan or (b) fixed monthly payments shall be made in approximately the same amount during the term of the loan, which payments shall first be applied to interest and the balance to principal. The aggregate balance of principal of such loans outstanding at any one time shall not exceed twenty-five percent of the assets of a credit union. Loans made under this paragraph may be secured by a first or subordinate mortgage on the real estate to be improved.