Because the flat rate model provides for the lowest cost of service it is the most common model used for residential customers. Locally, in Clark County there are a total of 7 sewer utilities and 4 utilize a flat-rate system for their residential customers, while 3 have consumptive-based rates. The utilities that offer a consumptive based bill almost always still have a “minimum” charge or “base rate”. Also, they are almost always a joint utility, offering both water and sewer service.