The commission has the power, after hearing, upon notice, by order in writing, to require every public utility as defined in §65-4-101 to carry for the protection of stockholders, bondholders or holders of securities a proper and adequate depreciation account in accordance with such rules, regulations, and forms of account, as the commission may prescribe. The commission shall have power to ascertain and determine, and by order in writing, after hearing, fix proper and adequate rates of depreciation of the property of each public utility, and each public utility shall conform its depreciation accounts to the rates so ascertained, determined, and fixed, and shall set aside the moneys so provided for out of earnings and carry the same in a depreciation fund. The income from investments of moneys in such fund shall likewise be carried in such fund. This depreciation fund shall not be expended otherwise than for depreciation, improvements, new constructions, extensions, or additions to the property of such public utility, unless the commission shall by order in writing give permission to the public utility to divert the fund to purposes other than those named in this section.