Annual cost-of-ownership means the sum of the annual capital costs and annual operation costs of the support structure which shall be the average costs of all similar support structures owned by the public utility. The basis for computation of annual capital costs shall be historical capital cost less depreciation. The accounts upon which the historical capital costs are determined shall include a credit for all reimbursed capital costs of the public utility. Depreciation shall be based upon the average service life of the support structure. As used in this definition, “annual cost of ownership” shall not include costs for any property not necessary for a pole attachment.