When complaint has been made to the commission that any public service company has charged an amount for any service rendered in excess of the lawful rate in force at the time such charge was made, and the same has been investigated and the commission has determined that the overcharge allegation is true, the commission may order that the public service company pay to the complainant the amount of the overcharge so found, whether such overcharge was made before or after the filing of said complaint, with interest from the date of collection of such overcharge.