When complaint has been made to the commission concerning any rate, fare, toll, rental or charge for any product, or commodity, furnished or service performed by any public utility, and the commission has found, after investigation, that the public utility has charged an excessive or discriminatory amount for such product, commodity or service, the commission may order that the public utility make due reparation to the complainant therefor, with interest from the date of collection: provided, no discrimination will result from such reparation.