The Los Angeles Department of Water and Power has agreed to pay millions of dollars in refunds for six years’ worth of allegedly inflated sewer charges. … At issue is how the DWP estimates its customers’ use of the city sewers. With no meters on houses directly measuring sewer use, the utility has to estimate that amount. That estimate relies on a crucial assumption: that the sewers receive all the DWP-delivered water a customer uses indoors and none of the water used outdoors.