(a) If the commission, on its own motion or on complaint by an affected person, after reasonable notice and hearing, finds that the existing rates of a public utility for a service are unreasonable or in violation of law, the commission 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 utility.
(b) The rates established under Subsection (a) constitute the legal rates of the public utility until changed as provided by this title.
(c) This section does not apply to a company electing under Chapter 58 or Chapter 59 except as otherwise provided by those chapters.