Every public utility shall carry a proper and adequate depreciation or deferred maintenance account, whenever the public utilities commission, after investigation, determines that a depreciation account can be reasonably required. The commission shall ascertain, determine, and prescribe what are proper and adequate charges for depreciation of the several classes of property for each public utility. The charge for depreciation shall be such as will provide the amount required over the cost and expense of maintenance to keep the property of the public utility in a state of efficiency corresponding to the progress of the art or industry. The commission may prescribe such changes in such charges for depreciation as it finds necessary.