The Corporation Commission is hereby vested with the power of a court of record to determine: First, the amount of refund due and any interest owing upon such refund in all cases where any public service corporation, person, or firm, as defined by the Constitution, charges an amount for any service rendered by such public service corporation, person, or firm, in excess of the lawful rate in force at the time such charge was made, or may thereafter be declared to be the legal rate which should have been applied to the service rendered if no legal rate was in force at the time such charge was made; and, second, to whom the overcharge should be paid.