Anything to the contrary herein notwithstanding, in the situation where a single water service, with 1 water meter, is used to supply multiple household or dwelling units, the amount of the monthly billing for that water service shall not be less than an amount determined by multiplying the total number of household or dwelling units (whether occupied or not) located on the property receiving the water service by the minimum charge per month, as established in § 51.003. At no time shall the multiple unit base fee be less than the monthly readiness to serve charge for equivalent sized meters, as determined in the water rate fee schedules applying to the respective water systems.