N.H. Rev. Stat. Ann. § 378:7

Whenever the commission shall be of opinion, after a hearing had upon its own motion or on motion of the department of energy or upon complaint, that the rates, fares or charges demanded or collected, or proposed to be demanded or collected, by any public utility for service rendered or to be rendered are unjust or unreasonable, or that the regulations or practices of such public utility affecting such rates are unjust or unreasonable, or in any wise in violation of any provision of law, or that the maximum rates, fares or charges chargeable by any such public utility are insufficient, the commission shall determine the just and reasonable or lawful rates, fares and charges to be thereafter observed and in force as the maximum to be charged for the service to be performed, and shall fix the same by order to be served upon all public utilities by which such rates, fares and charges are thereafter to be observed. The commission shall be under no obligation to investigate or hear any rate matter which it has investigated within a period of 2 years, but may do so within said period at its discretion.