There is hereby imposed and levied 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 the city, such public utility service. Such tax shall be in the amount of 2% 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 such purchaser by the seller, and shall be paid by the purchaser to the seller at the time the purchase price or such charge shall 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 $20,000 in any given calendar month, to any single purchaser, no tax shall be imposed for such additional purchase, use, or consumption in excess of the amount of $20,000. In the event more than one public utility shall furnish the identical public utility service to the same purchaser, such 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 such public utility service.