Section 15. A gas or electric company, under the supervision of the department, selling, offering for sale or issuing, bonds, debentures, notes or other evidences of indebtedness, exclusive of stock, payable at periods of more than five years after the date thereof, shall invite proposals for the purchase thereof by advertisements in two or more daily newspapers, if there be such, in the city or town where the company maintains its principal business office, as reported in its annual return filed under section eighty-three, and in two or more newspapers published in Boston. It may, however, reserve the right to reject any and all bids. It need not invite such proposals with respect to an issue of bonds, debentures, notes or other evidences of indebtedness not exceeding one million dollars in face amount. The department may grant an exemption from part or all of the requirements of this section whenever it finds after a public hearing that the granting of such exemption is in the public interest. If no such proposal is accepted, it may sell the whole or any part of said bonds, debentures, notes or other evidences of indebtedness to any person or corporation in such manner, at such times, and upon such terms as its directors shall determine.