1. Summary investigations. The commission may on its own motion, with or without notice, summarily investigate when it believes that:
A. A rate or charge is unjust or unreasonable;
B. A service is inadequate or cannot be obtained; or
C. An investigation of any matter relating to a public utility should for any reason be made.
2. Formal investigation. If after the summary investigation, the commission is satisfied that sufficient grounds exist to warrant a formal public hearing as to the matters investigated, it shall give the interested public utility written notice of the matter under investigation. Seven days after the commission has given notice, it may set a time and place for a public hearing. The hearing shall be held in accordance with section 1304.