When complaint has been made to the commission concerning any rate or charge for any product or commodity furnished or service performed by any public utility, and the commission has found, upon a hearing after notice given as required by this title, that the public utility has charged an excessive or discriminatory amount for such product, commodity, or service, in excess of the schedules, rates, and tariffs on file with the commission, or has discriminated under said schedules against the complainant, the commission may order that the public utility make due reparation to the complainant therefor, with interest from the date of collection, if no discrimination will result from such reparation.