Some customers have water services that are unmetered. These would include private fire hydrants, fire suppressant systems and sprinklers. Some utilities have flat charges that are billed each month so that the service is available if needed. Usually these charges are based on the number of hydrants, size of suppressant system or sprinkler heads in service. Municipal fire hydrant maintenance may be charged to the city’s general fund. One very important thing to remember is that utilities should derive 90 to 95 percent of their revenues through user charges.