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You say the builder was paying it? If the builder paid it, your reimbursement would be to the builder not to the utility company unless they refunded the money to the builder. If the utility company refunded the money to the builder and they can prove that you did use that much in utilities after their audit, then even if it was their mistake, you would by contract law and the equitable theory of "quantum meruit" (for the service given) and also under the equitable theory of unjust enrichment, you would owe the money I am sorry to say. However, because it was their error they should not charge you any late fees, interest or penalties and if they are trying to do so then you have grounds legally to fight those and have them removed.
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