Temporary rates so fixed, determined, and prescribed under this subdivision shall be effective until the final determination of the rate proceeding, unless terminated sooner by the commission. In every proceeding in which temporary rates are fixed, determined, and prescribed under this subdivision, the commission shall consider the effect of such rates in fixing, determining, and prescribing rates to be thereafter demanded or received by such public utility on final determination of the rate proceeding. If, upon final disposition of the issues involved in such proceeding, the rates as finally determined are in excess of the rates prescribed in such temporary order, then such public utility shall be permitted to amortize and recover, by means of a temporary increase over and above the rates finally determined, such sum as shall represent the difference between the gross income obtained from the rates prescribed in such temporary order and the ross income which would have been obtained under the rates finally determined if applied during the period such temporary order was in effect.