The Los Angeles Department of Water and Power owes its customers at least $67.5 million in refunds and credits after the utility overbilled them, an independent monitor has concluded. The latest estimate represents a significant increase from the proposed $44-million class-action settlement announced in August 2015. The DWP has been plagued by a faulty computer billing system launched in 2013 that overcharged tens of thousands of customers while failing to bill others at all. Officials said this week that after a year of working with the independent monitor and revising the settlement agreement, customers can expect to get refunds as early next summer.