N.H. Rev. Stat. § 365:29
On its own initiative or whenever a petition or complaint has been filed with the commission covering any rate, fare, charge, or price demanded and collected by any public utility, and the commission has found, after hearing and investigation, that an illegal or unjustly discriminatory rate, fare, charge, or price has been collected for any service, the commission may order the public utility which has collected the same to make due reparation to the person who has paid the same, with interest from the date of the payment. Such order for reparation shall cover only payments made within 2 years before the earlier of the date of the commission's notice of hearing or the filing of the petition for reparation.
RSA 365:29
1917, 76:3. PL 238 :27. RL 287:28. 1951, 203:11 par. 29. 2008, 309:1, eff. Aug. 31, 2008.