The commission may grant to consumers a rebate or discount for the prompt payment of water rent or rates within a specified time after such water rent or rates shall become due. Such rebate or discount shall not exceed the sum of ten cents on each one dollar or fraction thereof due for water rent or rates, and no rebate or discount shall be granted unless such rents or rates be paid within thirty days after the same shall become due. The sum so granted as a rebate or discount shall be fixed and determined by resolution of the commission.