If, after hearing, the Commission finds any existing or proposed rate unjust, unreasonable or unjustly discriminatory, or in any wise in violation of law, the Commission shall determine the just and reasonable rate to be charged or applied by the utility for the service in question, and shall fix the same by order to be served upon the utility; and such rate shall thereafter be observed until changed, as provided in this chapter. In determining the just and reasonable rate to be charged, the Commission shall consider the revenue needs of the utility, its past and projected rates of return on its rate base, or, when appropriate, its operating ratio.