(a) If the regulatory authority, after reasonable notice and hearing, on its own motion or on complaint by any affected person, finds that the existing rates of any utility for any service are unreasonable or in any way in violation of any law, the regulatory authority shall determine the just and reasonable rates, including maximum or minimum rates, to be observed and in force, and shall fix the same by order to be served on the utility. Those rates constitute the legal rates of the utility until changed as provided in this chapter.
(b) If a utility does not itself produce that which it distributes, transmits, or furnishes to the public for compensation, but obtains it from another source, the regulatory authority may investigate the cost of that production in any investigation of the reasonableness of the rates of the utility.