Section 18. Except as otherwise provided in this section, shares of the capital stock of a gas or electric company shall be offered at such price, not less than par, as its directors may fix. The vote of the department, as provided in section fourteen, as to the amount of stock reasonably necessary for the purpose for which the issue of such shares has been authorized, shall be based on the price fixed by the directors unless the department deems that such price is so low as to be inconsistent with the public interest, in which case it may fix the price at which such shares may be issued. If it deems that such shares cannot be disposed of at or above the par value thereof, it may, notwithstanding the provisions of this or any other section, authorize the issue of such shares at a price less than the par value thereof and with such requirements for the amortization of the discount as it may deem necessary in the public interest. Its determination shall be made part of the vote of the department as provided in section fourteen, and shall be certified and recorded as therein prescribed.