All interest payable under this Section 13.3 shall be calculated pursuant to 18 C.F.R. § 35.19a(a), as such regulation (or any successor thereto) is in effect during the period during which such interest is due. Amounts not paid when due to Owner or Distribution Company under this Agreement shall bear interest from the date such amount was due until the date of payment of such overdue amount. For the avoidance of doubt, as illustrated in Attachment H, if all or a portion of the amount to which such interest relates is later refunded pursuant to this Agreement, then, in calculating that refund, such interest shall not be included in the refund. Refunds of overpayments owed to Distribution Company by Owner under this Agreement shall begin to accrue interest on the amount subject to refund, as originally invoiced, from the earlier to occur of the due date or the date of payment of the monthly Invoices to which the overpayment relates and shall continue to accrue interest until the date of payment of such refund.