Mass. Gen. Laws ch. 164, § 122

Whoever, being engaged in the sale of electricity, maintains upon the premises of a customer for the purpose of determining the charge to be made for electricity supplied to him a meter, demand indicator or other mechanical device or appliance which is found upon examination and test, as provided in section one hundred and twenty, to register incorrectly as against such customer, shall refund to him such an amount as, if not agreed upon, shall, upon application of the customer and after opportunity given to the vendor to be heard, be determined by the department.