Every electrical utility may, and may be required to, charge annually as an operating expense a reasonable sum for depreciation and credit the same to a reserve account for such purpose. Such reserve account shall be charged only with plant retirements and expenditures made to restore depreciated property. But if the reserve thus created shall at any time in the judgment of the Commission be excessive, the Commission after due hearing shall make such order as will result in the credits to such reserve thereafter conforming to actual facts and conditions as ascertained by the Commission. The Commission may control or limit such depreciation reserve.