Section 58. There shall be fixed schedules of prices for gas and electricity, which shall not be changed oftener than once in three months. Any change shall take effect on the first day of a month, and shall first be advertised in a newspaper, if any, published in the municipality. No price in said schedules shall, without the written consent of the department, be fixed at less than production cost as it may be defined from time to time by order of the department. Such schedules of prices shall be fixed to yield not more than eight per cent per annum on the cost of the plant, as it may be determined from time to time by order of the department, after the payment of all operating expenses, interest on the outstanding debt, the requirements of the serial debt or sinking fund established to meet said debt, and also depreciation of the plant reckoned as provided in section fifty-seven, and losses; but any losses exceeding three per cent of the investment in the plant may be charged in succeeding years at not more than three per cent per annum. The gas and electricity used by the municipality for any purpose except street lighting shall be charged for in accordance with the prices in the fixed schedules. The gas and electricity used by the municipality for street lighting shall be charged for at a cost to be determined as follows: the sum of all operating expenses, interest on the outstanding debt, the requirements of the serial debt or sinking fund established to meet said debt, and also depreciation of the plant reckoned as provided in section fifty-seven, and losses, shall be the dividend; the kilowatt hours sold including those supplied for street lighting shall be the divisor, and the resulting quotient multiplied by the kilowatt hours supplied for street lighting shall be the cost to be charged to the municipality for street lighting. In lieu of the method of determining charges for electricity used by the municipality for street lighting, as set forth in the preceding sentence, electricity so used may be charged for at a cost in accordance with a street lighting schedule filed with and approved by the department.