There is hereby imposed upon each and every purchaser of a public utility service an excise tax upon the privilege of purchasing, using or consuming, within the corporate limits of this City, such public utility service. The tax shall be in the amount of two percent of the charge, exclusive of any federal or state tax thereon imposed upon the purchaser, made by the seller against the purchaser with respect to each public utility service, which tax in every case shall be collected by the seller and paid by the purchaser upon the amount of each periodic statement rendered the purchaser by the seller, and shall be paid by the purchaser to the seller at the time the purchase price or the charge all become due and payable under the agreement between the purchaser and the seller. The tax imposed and levied by this article is in addition to all other taxes imposed and levied by this City. In the event the amount of the charge for any single public utility service exceeds the sum of twenty thousand dollars ($20,000) in any given calendar month to any single purchaser, no tax shall be imposed for the additional purchase, use or consumption in excess of the amount of twenty thousand dollars. ($20,000). In the event more that one public utility furnishes the identical public utility service to the same purchaser, the purchaser shall be entitled to group the same as a single public utility service in calculating the amount of the charges in any calendar month for the public utility service. (1965 Code §20-9 )