(a) If the regulatory authority, on its own motion or on complaint by an affected person, after reasonable notice and hearing, finds that the existing rates of an electric utility for a service are unreasonable or in violation of law, the regulatory authority shall:
(1) enter an order establishing the just and reasonable rates to be observed thereafter, including maximum or minimum rates; and
(2) serve a copy of the order on the electric utility.
(b) The rates established under Subsection (a) constitute the legal rates of the electric utility until changed as provided by this title.