Ind. Code § 8-1-2-4

Sec. 4. Every public utility is required to furnish reasonably adequate service and facilities. The charge made by any public utility for any service rendered or to be rendered either directly or in connection therewith shall be reasonable and just, and every unjust or unreasonable charge for such service is prohibited and declared unlawful. The commission, in order to expedite the determination of rate questions, or to avoid unnecessary and unreasonable expense, or to avoid discrimination in rates between classes of customers, or, whenever in the judgment of the commission public interest so requires, may, for ratemaking and accounting purposes, or either of them, consider a single municipality and/or two (2) or more municipalities and/or the adjacent and/or intervening rural territory as a regional unit where the same utility serves such region, and may within such region prescribe uniform rates for consumers or patrons of the same class. Nothing in this chapter contained shall authorize any public utility during the remainder of the term of any grant or franchise under which it may be acting on May 1, 1913, to charge for any service, in such grant or franchise contracted, exceeding the maximum rate or rates therefor, if any, that may be fixed in such grant or franchise.