It shall be the duty of every public utility, upon written request by the customer, to determine the lowest rate applicable, provided that such public utility shall not be required to make such a determination for any single customer more frequently than annually. If the rate charged thereafter is not such lowest rate applicable, such public utility shall be liable to the customer for the amount of the difference between the amount paid by the customer and the amount that would have been paid if the customer had been charged the lowest rate applicable from and after the customer’s request; provided that the public utility may require and rely on written information from the customer relating to the customer’s expected demand for and use of the utility service where such information is relevant to the determination required hereunder. Where a contract for a specified period of time is lawfully required by the public utility, the rates prescribed by such contract shall be lawful during the term of such contract so long as they are the lowest applicable to the conditions of service specified in the contract, unless the actual conditions of service require the application of a higher rate. This section shall not be applicable to rates charged by any public utility prior to July 1, 1970.