German towns charge their residents fees for water, wastewater and waste collection. These fees are calculated, and the law limits the result. The municipal levies law of North Rhine-Westphalia says the expected fee revenue should not exceed the expected costs of the facility. The costs are those that may be counted under business accounting principles. If a town collects more than its costs, it must offset the surplus in the fees of the following years. Kostenüberdeckungen am Ende eines Kalkulationszeitraumes sind innerhalb der nächsten vier Jahre auszugleichen. A surplus at the end of a calculation period must be balanced within the next four years. The principle is called the Kostendeckungsprinzip, the cost-coverage principle.