In any proceeding involving the rates of a public utility brought either upon motion of the commission or the department of energy or upon complaint, the commission may, after reasonable notice and hearing, if it be of the opinion that the public interest so requires, immediately fix, determine, and prescribe for the duration of said proceeding reasonable temporary rates; provided, however, that such temporary rates shall be sufficient to yield not less than a reasonable return on the cost of the property of the utility used and useful in the public service less accrued depreciation, as shown by the reports of the utility filed with the commission and the department of energy, unless there appears to be reasonable ground for questioning the figures in such reports.